Module 2 Introduction

Water

“Recirculating systems are mechanically sophisticated and biologically complex. Component failures, poor water quality, stress, diseases, and off-flavor are common problems in poorly managed recirculating systems” (Masser, Rakocy, & Losordo, 1999, p. 1). The focus of this module is water quality. As an aquaponics operator, you “must have a good understanding of water quality chemistry, be able to accurately use meters and chemical test kits, and take the proper actions to quickly correct problems” (Damron, 2013, p. 502). You will complete readings and exercises that will allow you to outline the nitrogen cycle and explain its importance to basic aquaponics function; monitor & maintain aquaponic water quality for important characteristics such such as pH, oxygen, conductivity and ammonia; supplement with micronutrients to maintain nutrients such as iron, potassium and/or calcium when or if required; and identify the basic kits and meters available to measure water quality. You will also write a quiz in this module.

Learning Outcomes

Upon completion of this module, you should be able to

Determine which water quality parameters need to be monitored and where in the system they need to be measured
Select appropriate methods/techniques that can be used to measure water quality
Identify basic kits and meters available to measure water quality

An Introduction to Water Quality

Maintaining water quality is an important element of a healthy aquaponics system. In the aquaponics system, there are three key organisms: the fish, the plants, and the bacteria. Each of these organisms is, in some way, dependent on the others for survival. Later in this module, you will learn about the nitrogen cycle. This cycle is closely monitored through frequent water testing.

In an aquaponics system, source water may be well water, municipal water, and/or rain water. It is important to note that municipal water must be dechlorinated before it can be used in an aquaponics system. Though tap water may be safe for humans to drink, the chlorine would inhibit bacterial growth in an aquaponics system, and besides this fish are also sensitive to chlorine and can not live in chlorinated tap water for long. Using surface water is also not recommended as it harbors many organisms (parasites, disease, viruses, etc.) that you do not want in your aquaponics system. It is important to obtain a water quality profile of your source water before starting your system, so you know of any potential concerns.

Important water quality parameters include the following:

Dissolved oxygen
Nitrogen (ammonia, nitrite, nitrate)
pH and alkalinity
Carbon dioxide
Total dissolved solids and conductivity
Settleable and suspended solids
Water temperature
Nutrients

You will explore each of these parameters separately as you progress through this module. However, it is important that you remember that many of these parameters influence each another. The aquaponics system is highly integrated. The combination of all these water quality parameters helps you to assess the health of your aquaponics system on a chemical level. When the level of one parameter changes, for example pH, it will impact the levels of other water parameters. Changing water quality parameters influences the fish, the plants and the system’s microbial community, so if possible, always remember to make gradual changes to water chemistry. Avoid large or quick alterations and your system will perform much better. Water quality monitoring, in combination with your inspection and assessment of your plants and fish, will help you maintain a healthy system.

As you work your way through this module, you will learn about a variety of meters and testing procedures. Generally, water quality is measured using test kits, meters, spectrophotometers, or even certified labs.

In general, when selecting any of the methods that can be used for testing water quality in aquaponics, be reminded that each has its benefits and challenges. For instance, water quality meters are very precise testing instruments that can give you a quick result saving you time, but they are also quite expensive and up to now can’t test for everything that is required in aquaponics. Water chemistry kits on the other hand can test for a wide variety of water quality parameters (all those needed in aquaponics), are much lower cost than meters, but provide moderate accuracy, have reagents that have a limited shelf life and take much more time to obtain a result. Test strips are the least expensive and are quick to obtain a result with, but they must be kept dry, have a limited shelf life and moderate to low accuracy. Generally a combination of test methods can be selected depending upon your needs, but in time the better you know your system the less frequently you need to test it and the fewer water quality parameters you need to test for.

Dissolved Oxygen

Dissolved oxygen (DO) is essential for efficient nitrification, encourages beneficial bacteria and is important for fish and plant survival and growth. Dissolved oxygen should be available at all locations and at all times in your aquaponics system. To maintain your fish, no matter what species, you should aim to keep DO levels above 5 mg/liter. This is not to say that all fish need this amount of dissolved oxygen in the water at all times, but it is a good rule of thumb and provides a level so you can have time to react if you find your oxygen is dropping below this level. You also want enough oxygen to allow for fish to grow as growing fish have a higher metabolism, and consume more oxygen. Consequently when you feed fish, their metabolism and activity will increase and they will consume more oxygen while they are acquiring and digesting the feed.

Oxygen is one of the most important water quality parameters to maintain in your system because all three aquaponics components (plants, fish, and bacteria) require oxygen to live. All three of these components consume oxygen to live and grow, so the more plants, the more fish, and the more bacteria you have in your system the greater the oxygen demand and the more dissolved oxygen you have to provide. Some of you may be thinking this is not right because plants photosynthesize to produce oxygen, not consume it, but this is only true for the foliage, not true for the roots. And the foliage only photosynthesises when there is light, so at night they too respire.

Besides the fish, plants, and bacterial density in your system, other factors also influence the DO concentration. Water holds less oxygen at higher temperatures, so if you are raising tilapia at 28-29°C versus 24-25°C, your oxygen levels may be lower. If these levels are below 5 mg/l, your options are to increase aeration, reduce the number of fish, or switch to oxygenation. One major factor that often causes DO problems in aquaponics systems after some time of trouble-free operation is the accumulation of suspended solids. Suspended solids, as they decompose, consume oxygen. If allowed to accumulate in your system, suspended solids will progressively consume more and more oxygen. Now an argument here could be that you want some suspended solids in your system because as they decompose they release minerals and nutrients that your plants can make use of. This is all true, but the control of solids is vital to long-term functioning of your system. Too much deposition of solids in the system have caused many aquaponics systems to fail.

The minimum oxygen requirements for your system are dependent on two things:

Genetics
- Different fish species have different oxygen requirements; for example, carp have a high tolerance to low oxygen (affinity and carrying capacity of hemoglobin, accessory organs for oxygen exchange, etc.)
Metabolic rate
- Relative metabolic rate is higher in smaller fish. Increased stress increases metabolism, which in turn increases oxygen demand. Increased temperature and activity are also intimately related to metabolic rate.

The two major sources of natural oxygen in water are from surface exchange with the atmosphere and photosynthesis. The reduction of oxygen in water can be a function of a variety of factors.

In general oxygen concentrations less than 3 ppm (parts per million) will support minimal life. 3-5 ppm can support most invertebrates and some fish. Oxygen concentrations greater than 5 ppm will support most all species. This level is considered the absolute minimum safe concentration for aquaculture (try to maintain oxygen saturation at 100%). In a pond situation there may be a daily cycling of high and low oxygen concentrations and/or a significant difference between DO at the surface of the water and DO at the bottom, but this is not what we wish for in an aquaponics system. You will want to keep ample DO throughout your system at all times.

An important definition regarding dissolved oxygen is biochemical oxygen demand (BOD). BOD is the amount of dissolved oxygen that is demanded (or needed) by the aerobic biological organisms (microbes) in your aquaponics system to break down the organic material (e.g., fish fecal waste) that is present in the water. In other words, it is the amount of oxygen required for microbial metabolism. The more solids that are present in your aquaponics system, the more bacteria that will colonize it and therefore the greater respiration or oxygen being demanded. Although BOD can be measured as part of your water quality testing, it is usually simpler to just measure DO and keep an eye on your TSS (Total Suspended Solids).

Aeration versus Oxygenation

This may seem like a simple decision, but there are more considerations other than how you are going to achieve the desired dissolved oxygen in your system. Aeration involves using atmospheric air to supply the oxygen in your system. This is achieved by relatively simple pumps like those used in an aquarium. It usually requires an air diffuser stone which breaks the air into fine bubbles to enhance diffusion into the water. Since air is only about 20% oxygen, much of what you are bubbling into your water is not what you aim to dissolve. Oxygenation on the other hand uses concentrated oxygen as a source to bubble into your water. Greater DO concentrations can be achieved with oxygenation than with aeration. Oxygenation is usually desired when you have a large aquaponics system, or have low air circulation, such that humidity becomes elevated. High air humidities negatively impact plant productivity and can lead to certain plant diseases. If this occurs, you will have to consider switching from aeration to oxygenation.

Dissolved Oxygen Readings

(http://www.gkscientifics.com/ (Links to an external site.))

(http://www.gunapris.net/ (Links to an external site.))

There are two oxygen readings:

% saturation: This tells you the status; in other words, could it or should it be better? Water always wants to achieve 100% saturation, but because there are a number of biological and physical factors which either consume/produce oxygen, or influence oxygen’s solubility in water, the percent saturation is always in flux. Percent saturation lets you know how much more oxygen could be dissolved under the conditions of your system (e.g., if saturation is 50% you know that oxygen levels could be doubled and also that half of the oxygen you are supplying is being consumed).
Concentration (mg/l): this measurement indicates the actual amount of oxygen dissolved in the water. You can often use absolute values of DO concentration to ensure adequate levels are available for the fish, plants, and bacteria in your aquaponics system.

Dissolved oxygen levels are achieved differently in the different types of aquaponic systems.

In ebb and flow media bed systems, the bacteria that colonize the media are exposed to atmospheric oxygen during the ebb or dewatering phase.
In the NFT system “The thin film of water that flows through NFT channels absorbs oxygen by diffusion, but dense plant roots and associated organic matter can block water flow and create anaerobic zones, which precludes the growth of nitrifying bacteria and further necessitates the installation of a separate biofilter” (Rakocy et al., 2006, p. 7).
In a DFT system, the beds of water are covered by the polyethylene rafts that support the plants. Sufficient nitrification occurs if the solids are removed from the system before they enter the plant bed. As the roots are fully immersed in water at all times, adding dissolved oxygen to the plant bed in the DFT may be a necessary step if DO decreases to below 3 mg/l in this location.

DO can be measured chemically, but preferably, it is measured with a meter. Chemical methods may be less expensive in the short-term, but not in the long-term. These tests also take much longer to perform (10–15 minutes). Dissolved oxygen should be measured frequently in a new system, but once your aquaponics system stabilizes and you become more familiar with it, monitoring for DO can be scaled back.

Nitrogen (Ammonia, Nitrite, Nitrate)

All animals excrete waste, including nitrogenous waste products, such as urea, uric acid, and ammonia. Fish excrete nitrogenous waste as ammonia from their gills. Ammonia also enters the aquaponics system from fish urine and fecal waste and uneaten food.

Ammonia, is present in two forms, as NH₄+ (ammonium) and as NH₃ (unionized ammonia). The NH₃ form of ammonia is extremely toxic to fish and when it accumulates, it can cause serious damage. In high enough concentration, ammonia inhibits growth and development and is toxic to gill epithelium. High ammonia can kill fish. Therefore, handling ammonia levels is a major consideration for aquaponics. The maximum acceptable level for unionized ammonia should be 0.01 mg/L. 0.06 mg/L is capable of killing certain species of fish, and if prolonged, may reduce growth and reduce the fishes ability to use oxygen.

“Ammonia will accumulate and reach toxic levels unless it is removed by the process of nitrification (referred to more generally as biofilitration), in which ammonia is oxidized first to nitrite, which is also toxic, and then to nitrate, which is relatively non-toxic” (Rokacy et al., 2006, p. 6). In aquaponics, the main function of the biological filter (also known as the biofilter) is to transform toxic ammonia into a non-toxic chemical species.

NH₃ / NH₄ ⟶ NO₂ ⟶ NO₃

There are two groups of naturally-occurring bacteria that facilitate the nitrification process: Nitrosomonas and Nitrobacter. In short, the biofilter is a microbiological, living component of your aquaponics system that facilitates the chemical conversion of ammonia to nitrate (nitrification). The biofilter has a large surface area on which the bacterial species reside and do their thing to improve water quality. To speed preparation of the biofilter when starting your aquaponics system, you can seed the filter (using Bacta-Pur for example). In order to maintain the biofilter and its operation, oxygen must be present at all times, and total suspended solids getting to the biofilter should be limited.

The sum of the two forms of ammonia (NH₄+ & NH₃) is called total ammonia-nitrogen (TAN). Unionized ammonia (NH₃) is toxic to tilapia at approximately 1 mg/liter. Ammonia is not toxic as an ion NH₄+ (ammonium). Ammonia exists as a ratio of NH₄+:NH₃. This ratio is dependent on pH and temperature. At pH 7.0 or below, most ammonia (>95%) exists as the ion (NH₄+). The goal for aquaponics is to maintain TAN at a level less than 3 mg/liter.

Note: You will explore the nitrogen cycle in more depth in Module 3: Microbiology.

Nitrifying bacteria grow as a film (biofilm) on the surface of inert material or they adhere to organic particles. A biofilter in an aquaponics system contains media that has large surface area so that the nitrifying bacteria have lots of opportunity to grow. You will explore biofiltration in more depth on the following page.

Ammonia Removal

TAN is removed by the nitrifying bacteria (Nitrosomonas sp.) and plants (first stage). Ammonium is oxidized to nitrite (NO₂). This first stage in nitrification, the conversion of NH₃ to NO_2,liberates hydrogen ions to the water, which erodes alkalinity and causes the pH to drop. Nitrifying bacteria grow on underwater surfaces (fixed film) and on suspended organic particles, and the process of nitrification is optimized at levels of high DO and low levels of organic matter.

The following measures can be taken to alleviate total ammonia if your aquatic system begins to experience high ammonia levels:

Reduce feeding rates and/or maximize efficient use of protein in the diet
Increase the supply of fresh water
Reduce fish stocking density of system
Stimulate phytoplankton growth with phosphates
Lower pH
Dry ponds can be treated with calcium oxide to hasten NH₃ loss.

From an aquaponics standpoint, some of these measures either don’t apply (eg. pond treatment) or would not be recomended. Not overfeeding your fish and reducing total suspended solids can do a lot in reducing TAN in your aquaponics setup. Total ammonia entering a system varies with the amount of feed used and the percentage of protein in the diet. Channel catfish, for example, excrete about 20 g of ammonia/day/kg of ration fed, whereas salmon excrete 25-35 g ammonia/day/kg of feed. A salmon diet has more protein than a catfish diet and so more ammonia will be excreted.

Though plants can use urea, ammonia or nitrate as a nitrogen source, it is the toxicity of ammonia and urea to fish that requires the need to break it down into the less toxic nitrogen source nitrate.

The amount of unionized ammonia present in a system can be arrived at mathematically and through the use of tables. Either way you must know the pH and temperature of your water.

Nitrite Removal

Nitrite (NO₂), like unionized ammonia, can be viewed as being toxic to fish. In high enough concentrations nitrite causes methemoglobinemia, the conversion of hemoglobin to methemoglobin in the blood, which inhibits the transport oxygen, and so, fish die of hypoxia. Nitrite is an intermediate product of the biological oxidation of ammonia to nitrate (NO₃) through nitrification.

Toxicity varies depending on the fish species and is influenced by the presence of other anions (Cl^– and Ca⁺). For example, 13 mg/L NO₂ is toxic to catfish and 0.3 mg/L NO₂ is toxic to salmonids. For talapia, a common fish raised in aquaponics, it is ideal to maintain nitrite-nitrogen below 1 mg/liter. Increased anion content increases tolerance to high nitrite. A maximum chronic exposure level of less than 0.1 mg/L is a suggested guideline.

Nitrate

In the second stage of nitrification, nitrite (NO₂) is oxidized to nitrate (NO₃) by a second nitrifying bacterial group (Nitrobacter sp.). This stage of nitrification also erodes alkalinity, produces hydrogen ions, and lowers the waters pH. Nitrate is relatively non-toxic to fish unless it is allowed to build up to high concentrations in the system. Unlike recirculating aquaculture systems that only have fish in the system, aquaponics systems have plants that absorb the nitrates and other nutrients from the water. By the time the system water is pumped back to the fish tank, the ammonia has been transformed to nitrate and the nitrate has been absorbed by the plants.

Important Note: The information above covers the basic process of nitrification. It is important to remember that the living microbes in your aquaponics system must always be maintained. This, as described above, needs the continued presence of oxygen along with the minimization of the suspended solids being introduced into the biofilter. In addition to these, any dramatic change to water chemistry (i.e., pH, temperature) will negatively impact your aquaponics system’s microbiology. Any modifications to water chemistry should be done gradually. In turn, monitoring on a regular basis and not letting water quality parameters to fluctuate widely is important for getting to know and understand your aquaponics system, and vital to its microbial integrity.

Biofiltration

The removal of ammonia and nitrite is referred to as biofiltration/nitrification. As you have read, nitrifying bacteria grow as a film on the surface of any inert material in your system. The amount of biofiltration you need for your system is directly related to how much feed you give your fish. Higher densities of fish, greater weight of fish, or higher protein levels in the diet will all increase the ammonia released and conversely the greater the bacterial surface area you’ll need to convert ammonia and nitrite to nitrate. Sometimes the aquaponics system itself has enough surface area (i.e., media-based systems) to facilitate the bacterial attachment and growth, such that a distinct biofilter is not required. An aquaponics biofilter should be sized appropriate to the fish loading density, so if you plan on having a high fish stocking density you will need the biofilter surface area close to the recommendations needed for recirculation aquaculture systems. Although much of this information is available online and becomes complex, in general, under ideal conditions if using a moving bed biofilter, each square meter of media can convert the ammonia produced from feeding 9 kg per day of feed composed of 40% protein. In DFT aquaponic systems, the combination of plants and raft surface area can be the biofilter, but often, if the system’s surface area is not extensive enough, additional biofiltration surface area must be added to the system. A cautionary note here, having a biofilter is like insurance, so if you decide to forgo one, you have nothing to fall back on if nitrification is hampered. In media-based aquaponics systems, all surface area provides space for nitrifiers. Depending on the goals for a specific system, a biofilter may be used to:

Remove ammonia
Remove nitrites
Remove dissolved organic solids
Add oxygen
Remove carbon dioxide
Remove excess nitrogen and other dissolved gasses

If additional biofiltration is required, a moving bed filter with a type of high surface area bead is recommended (i.e., kaldnes media).

If you decide to design a larger aquaponics system, perhaps one for commercial operation, you will need an appropriately intensive biofilter. There are numerous biofilter options available. Some examples are listed below. You can do additional research on each of these on the Internet.

Rotating biological contactors
Expandable media filters
Fluidized or moving bed filters
Packed tower filters

Biofilter Establishment

Depending on water temperature, with cooler water temperatures taking longer, biofilters require at least 4 to 6 weeks, sometimes 6 to 8 weeks, for sufficient bacterial populations to develop and establish. To initiate the biofilter, fish stocking density and feed rate should be low (<1 kg/m³), otherwise ammonia could build up to toxic levels and kill your fish. Alternately and recommended, you can start your biofiter before adding fish by using finely ground fish feed, or ammonium chloride and keep ammonia levels at between 1-2 mg/l. At this stage, it is important to measure ammonia, nitrite, and nitrate daily. As the first stage of the biofilter begins to establish, ammonia levels will begin to decline, and correspondingly nitrite levels will increase. Remember nitrite is also toxic to fish, so it is important to not let Total Ammonia Nitrogen (TAN), or nitrite nitrogen (NO₂-N) exceed 5 mg/liter. The second stage of the biofilter is established when TAN and nitrite levels decline, and correspondingly nitrate levels increase. At this point, TAN should be low, NO₂-N should be declining, but must still be monitored.

In order to eliminate start-up cost and operational expenses, an aquaponics system could be designed so that the hydroponic subsystem also serves as the biofilter (Rakocy, et al., 2006). In some systems, such as an ebb and flow system, granular hydroponic media (e.g., gravel, perlite) can serve as the sole biofilter, offering enough surface area for nitrification to occur. Media can clog though, resulting in too much ammonia staying in the system. In fact, if too much organic matter exists and decays, the media bed can actually start producing ammonia instead of removing it. After the media is washed, a solids removal device is necessary. Alternatively, the fish stocking density and/or feeding rate could be reduced. In some systems, if the biofilm layer becomes thick, it is necessary to remove some of the biofilm that accumulates so the biofilter can perform sufficiently. This will avoid media clogging, prevent problems with water flow, and reduce the oxygen demand of your system.

Note: In the above reading they suggest using sodium bicarbonate as an alkalinity booster, which is fine for starting a biofilter for an aquaculture operation, but in aquaponics sodium (Na+) can buildup and become a problem for your plants. Instead of baking soda use calcium hydroxide, potassium hydroxide, or calcium carbonate to boost your alkalinity.

pH and Alkalinity

pH is a numeric scale for hydrogen ion activity and specifies if an aqueous solution as basic, neutral or acidic. The scale ranges from 0 to 14, with a pH of 7 being neutral, a pH less than 7 indicating an acid and a pH greater than 7 indicating a base. pH is known as the master variable because it influences many other water quality parameters; for example:

The pH level of water affects the efficiency of nitrification
pH determines the amount of NH₃ versus NH₄
Percent of NH₃ versus NH₄ affects the solubility of plant nutrients

The optimum pH range for nitrification is 7.0 to 9.0, with most nitrifying bacteria performing optimally at a pH greater than 7.5. Most plants prefer a pH under 6.5. When the pH is higher than 7.0, iron, manganese, copper, zinc, and boron (all essential nutrients) are less available to the plants. When the pH is lower than 6.0, phosphorus, calcium, magnesium, and molybdenum (all essential nutrients) decrease. Therefore in order to satisfy both plant and bacterial requirements, a compromise in pH must be reached. Maintaining a pH between 6.0 and 6.5 is ideal for aquaponic systems. However, an important thing to remember with adjusting pH is that a gradual slow adjustment is preferred. Drastic changes in pH (>0.5) will shock the bacteria, plants and fish in your aquaponics system and reduce nitrification, feeding and growth, or even lead to disease or death directly.

This image illustrates how the solubility of specific nutrients is affected by pH. For example, notice how iron is most soluble at a pH around 4.5 to 5.0. Iron supplements, therefore, are often necessary additions to aquaponic systems as the pH in an aquaponics system should ideally be between 6.0 and 6.5. At the other end of the scale, calcium is most soluble at a pH over 7.5, and so calcium supplements are also commonly required.

pH should be measured daily as it typically is always declining. Again, like for ammonia, nitrite and nitrate, measuring pH several times a day when you are establishing your aquaponics system will allow you to become more familiar with your system’s functioning. This level of water quality testing however is not required once you begin to know your system and become familiar with it. Remember, both the CO₂ generated by the fish and the process of nitrification happening in the biofilter contribute to this constant lowering of pH in the system. In order to maintain a pH between 6.0 and 6.5, an alkaline base should be added. You can alternate adding calcium hydroxide and potassium hydroxide as these bases will also contribute plant nutrients (Ca⁺ and K⁺). Sodium bicarbonate should be avoided as a base because of its contribution to elevated salinity.

Low pH

Sometimes aquaponics operators neglect to measure pH for several days. pH can quickly decrease to 4.5. At this level, nitrification stops and TAN concentrations can exceed 30 mg/liter. To rectify this issue, you would add base very slowly over several days. Adding a large amount of base at one time will convert the majority of TAN into the toxic form (NH₃), and will negatively impact your biofiltration and likely cause a fish kill. Also, allowing pH to decline to this degree would also impact your biofilters capacity to nitrify so expect ammonia and or nitrite levels to increase.

Alkalinity

Alkalinity is water’s ability to resist a drop in pH upon the addition of an acid. Therefore, to increase alkalinity of water a base should be added. The concentration of alkalinity is expressed as the equivalent concentration of calcium carbonate (CaCO₃). Alkalinity should be greater than 100 mg/liter as CaCO₃.

Carbon Dioxide

Sources of carbon dioxide in the aquaponics system include

Decomposition (mineralization)
Fish, plant and bacterial respiration
Atmospheric exchange (atmospheric exchange is the greatest source of free CO₂ for phytoplankton in natural aquatic systems)
Chemical reactions between acids and carbonate containing molecules

As it is a product of aerobic respiration, CO₂ levels can be extremely variable. For fish, 12 mg/L is considered a threshold range with 20 mg/L approaching a lethal level. At higher levels, fish become sluggish because CO₂ interferes with the fish’s ability to absorb oxygen from the water and carry it in the blood. Typically a high CO₂ level in your water is an indicator of low oxygen levels. In systems using diffused aeration there is enhanced agitation with water, so CO₂ buildup is not usually a problem because it is vented off to the atmosphere. When using oxygenation, there is reduced agitation and so the chance of having elevated CO₂ could potentially exist, even if the chance of this is slight. To decrease CO₂ in your water you can enhance diffusion via water agitation.

Carbon dioxide exhibits an equilibrium reaction with water as follows:

CO₂ + H₂O ⟷ H₂CO₃ ⟷ H⁺ + HCO₃ ⟷ 2H⁺ + CO₃

The form that carbon dioxide exists in at any one time is largely a function of pH.

Carbon dioxide is essential for

Photosynthesis
A buffering system
A source of carbon for organic molecules

Although elevated CO₂ is rarely a concern for aquaponics, be careful when using well or spring water, which is usually high in CO₂.

A clarification here is also required, since in aquaponics we are dealing with water and air and CO₂ in regards to fish, bacteria and plants. In aquaponics, although the plants roots are in the water, it is plants leaves that photosynthesize, and so it is the air CO₂ levels not the water CO₂ levels, which are important for photosynthesis to function properly.

Total Dissolved Solids and Conductivity

Dissolved Solids is the total concentration (in ppm (parts per million) or mg/L (milligrams per litre)) of all inorganic mineral salts in solution (CO₃, Cl, SO₄, NO₃, and salts of Na, Mg, K, Ca). Any compound or substance that remains in water after you pass it through a 2 um filter, which removes the suspended solids, is considered a part of the Total Dissolved Solids (TDS). Most natural waters that support fish have a TDS of 10-1,000 ppm, a wide range. Dissolved solids are important to aquatic life by way of osmoregulation. Osmoregulation is the ability of an organism to regulate its bodies water and ion balance. In water with low TDS there is a higher concentration of water and a lower concentration of ions outside of the organisms body, verses the concentration that is inside the body. In this situation, water will diffuse or flow into an organism and the organism must have the ability to get rid of the extra water and conserve its internal ion balance. As the waters TDS increases and gets more similar to the internal water and ion concentration of an organism, the easier osmoregulation becomes. Similarly, as the waters TDS increases beyond the organisms internal concentrations of water and ions, the organism must find ways of conserving water but getting rid of the ions that are perpetually trying to come in to balance the external and internal environments. In principal this explains why some fish can live only in fresh water and some can only live in sea water, but thankfully we don’t have to consider these TDS extremes in aquaponics. In aquaponics, maintaining and monitoring TDS is mostly important because you wish to maintain adequate nutrient levels in your water for the plants. Since TDS is not easily measured, conductivity is monitored instead.

Conductivity is a measure of a substance’s ability to pass an electrical current. This ability is directly related to the concentration of ions in the water, which come from dissolved salts and inorganic materials in the water, a large component of the TDS. The more ions that are present, the higher the conductivity of water. Conductivity is usually measured in micro- or millisiemens per centimeter (uS/cm or mS/cm). Since temperature influences conductivity, and for the sake of simplicity and being able to monitor, specific conductance is measured instead, which is the conductivity measurement corrected to 25° C. This is the standardized method of reporting conductivity. For an approximate conversion of conductivity (uS/cm) to TDS (mg/l) multiply the conductivity reading by 0.65.

Water hardness, a component of TDS, is a measure of the sum of all divalent cations dissolved in the water, of which calcium (Ca⁺⁺) and magnesium (Mg⁺⁺) are the most abundant. Other divalent cations which are minor contributors are iron, manganese and aluminum. Calcium is typically the most abundant cation found in fresh water. The ranges of water hardness in freshwater is broad and the general categories are stated in the table below.

Soft water	0-75 mg/liter
Moderately hard water	75-150 mg/liter
Hard water	150-300 mg/liter
Very hard water	300+ mg/liter

Settleable and Suspended Solids

Recall the discussion on total suspended solids in Module 1. The aquaponics system is highly influenced by the presence of suspended solids. Cumulatively, these are referred to as Total Suspended Solids (TSS). Any time you are raising a live specimen, you will have to deal with fecal waste, uneaten food, and any other matter that floculates, suspends or settles in your system. The more of this matter that can be removed, or retained but prohibited from reaching the biofilter and plant bed, the better success you will have in aquaponics. There are two basic strategies for handling suspended solids in aquaponics: 1) trap, keep, and retain suspended solids in the system such that they break down (mineralize) and provide nutrients, or 2) trap and eliminate suspended solids from the system. Retaining solids for mineralization is preferred, but must be planned for in the design as the solids must be processed properly. Looking into the future, a bioreactor; a controlled environment where the solid nutrients are efficiently mineralized and the liquid nutrients are fed back into the aquaponics cycle, is a probable component of tomorrows aquaponics systems. Other suspended solids, such as organisms (e.g., bacteria, fungi, and algae), can also accumulate in the system and may cause disruption if present in large enough quantities.

Fine suspended solids will accumulate in the water, causing the turbidity level to rise and water clarity to decrease. Turbidity in a liquid is much like smoke in the air. It is cloudiness or haziness that is caused by the individual solid particles and or colour of the water. Turbidity is a key water quality test.

Regarding your fish, suspended solids in your aquaponics system may

Provide a reservoir for bacteria
Increase the demand for oxygen in your system
Increase maintenance
Increase susceptibility to disease

Regarding your plants, fine suspended solids may collect in your aquaponics plant bed over time, requiring periodic removal. If left unattended suspended solids in your plant bed may

Decrease dissolved oxygen
Reduce plant growth
Increase plant stress and disease
Contribute to increased pests

Water Temperature

The metabolism of fish species is dependent on water temperature. Tilapia, for example, prefer 28-30°C for maximum growth. Tilapia feeding and growth slows dramatically under 21°C and reproduction stops. Tilapia die under 10°C. The incidence of disease in fish increases if water temperature changes drastically, or if fish are kept at temperatures outside of their comfort zone. Water temperature in aquaponics is usually a compromise between what is ideal for the fish and what is best for the plants. There are some cooler water plants that can be matched with cooler water fish, but plant production will most likely be slower. In our experience, although growth rate is reduced when growing plants at a cooler temperature, the taste of the produce is usually better.

Most often the temperature of the water in your aquaponics system is controlled by adjusting the air temperature of the room where your system resides. This works best for small systems and maybe even for some larger scale systems, but in some cases where the air temperature might get too warm, you can cool your water by burying the piping and your fish tank in the ground. The obvious negative aspect of this is access to piping for repairs and such is less efficient. You could also heat your water directly if air temperatures are cooling your water below appropriate temperatures for the fish and/or plants. Usually, aquaponics systems are not reliant on electric heaters or chillers directly heating or cooling the water, as these are expensive and increase the cost of operation. Instead, a better plan is to properly design and select an aquaponics system with appropriate fish and plant species to make better use of the climate at your location.

Vegetables prefer a temperature of 21-24°C. The following images show healthy plants and plants exposed to too much heat. NFT does not provide the same thermal buffering capacity as DFT, since you only have a trickle of water vs. a deep bed, something to consider if you are unable to finely control air temperature in your plant growing area.

(http://www.fabartdiy.com/fab-art-diy-pvc-gardening-ideas-and-projects/ (Links to an external site.))
Healthy basil in a NFT system.

Nutrients

The plants in an aquaponics grow bed require 16 essential nutrients to grow. Carbon dioxide and oxygen exist in the highest concentrations in the plants’ tissues. Remember these nutrients are all part of the total dissolved solids and cumulatively are monitored via measuring the conductivity of your system. Individual plant nutrients can be measured with chemical tests. Macronutrients, essential nutrients that are in relatively large quantities, and Micronutrients, essential nutrients that are in relatively smaller quantities include:

Note: You will explore the nutrients plants require in more depth in Module 5: Plants.

Macronutrients	Micronutrients
Carbon* (C)	Chlorine (C)
Oxygen* (O)	Iron** (Fe)
Hydrogen* (H)	Manganese (Mn)
Nitrogen (N)	Boron (B)
Potassium** (K)	Zinc (Zn)
Calcium** (Ca)	Copper (Cu)
Magnesium (Mg)	Molybdenum (Mo)
Phosphorus (P)
Sulfur (S)

*From CO₂ and H₂O ** Must usually be supplemented

In general: Phosphorous (P) – stimulates strong roots, Nitrogen (N) – shoots and Potassium (K) – flowers and fruits. A good nutrient ratio for seedling production is 9-45-15 (N-P-K), which promotes strong root formation.

Feeding Rate Ratios and Nutrient Supplementation

You will also explore this concept in more depth later in the course; specifically, in Module 4: Fish. From work conducted by Rakocy using his UVI aquaponics systems the optimum feeding rate is 60-100 g of fish feed per m² of plant growing area per day. Lennard takes a different approach to system loading and states it in the context of the total weight of fish needed to grow a specific number of plants. In this case however both the fish species and plant species are specific. For lettuce and staggered crops, for example, you should use a lower feed ratio. For fruiting plants and batch culture, you should use a higher feed ratio. These ratios generally supply 10 of the 13 nutrients required for plant crops.

However, nutrient supplementation is often required in aquaponics to achieve some of the required nutrients, especially if all solids are removed from the system and/or your system is newer. Calcium and potassium are two of the nutrients often required to be supplemented, but if using calcium hydroxide and potassium hydroxide as buffers to increase and maintain system pH, these essential nutrients can be supplemented concurrently. Basically you would alternate adding these supplements to your system in your pH maintenance regime.

In respect to iron, add 2 mg/liter of chelated iron (as iron) every three weeks or as needed. Use chelated iron designated as DTPA, EDDHA (specific for liquids, turns water red, lasts longer).

Measurement of Total Nutrient Concentrations

Use an electrical conductivity (EC) meter to measure nutrient levels. As previously stated, electrical conductivity is closely related to total dissolved solids (TDS), so as the amount of nutrients increases, so does EC. EC should range from 0.3 to 3.0 mS/cm. TDS should range from 200 to 2,000 mg/liter. EC and TDS are at the lower end of the range in aquaponic systems because nutrients are generated constantly. Below are visual examples of some of the available conductivity meters on the market.

The Sump

If you need to add supplemental nutrients to your system to meet your desired water quality parameters, you would do so in the sump. The sump is where the water is pumped back to the fish tank after it leaves the plant grow beds.

The sump plays an important role in the aquaponics system. In particular, a sump may be used in DFT, NFT, and ebb and flow systems. Rakocy et al. (2006) explain the sump’s function:

Water flows by gravity from gravel, sand and raft hydroponic subsystems to a sump, which is the lowest point in the system. The sump contains a pump or pump inlet that returns the treated culture water to the rearing tanks. If NFT troughs or perlite trays are located above the rearing tanks, the sump would be positioned in front of them so that water could be pumped up to the hydroponic component for gravity return to the rearing tanks. There should be only one pump to circulate water in an aquaponic system.

The sump should be the only tank in the system where the water level decreases as a result of overall water loss from evaporation, transpiration, sludge removal and splashing. An electrical or mechanical valve is used to automatically add replacement water from a storage reservoir or well. Municipal water should not be used unless it is de-chlorinated. Surface water should not be used because it may contain disease organisms. A water meter should be used to record additions. Unusually high water consumption indicates a leak. The sump is a good location for the addition of base to the system. Soluble base such as potassium hydroxide can cause high and toxic pH levels in the sump. However, as water is pumped into the fish rearing tank, it is diluted and pH decreases to acceptable levels. (p. 8)

Rakocy et al. (2006) expand upon the role of the sump by looking at an example, the UVI aquaponics system in St. Croix:

The UVI system has a separate base addition tank located next to the sump. As water is pumped from the sump to the fish-rearing tanks, a small pipe, tapped into the main water distribution line, delivers a small flow of water to the base addition tank, which is well aerated with one large air diffuser. When base is added to this tank and dissolves, the resulting high pH water slowly flows by gravity into the sump, where it is rapidly diluted and pumped to the fish rearing tanks. This system prevents a rapid pH increase in the fish-rearing tank. (p. 9)

Water Quality Management Options

Based on your water quality measurements, you may try various management techniques. The following table shows possible management options when specific water quality measurements are observed.

Observation	Possible Management
Low dissolved oxygen	increased aeration/oxygenation reduce feeding or increase number of feedings (same amount of feed over more feedings) reduce suspended solids
High carbon dioxide (above 20 ppm)	add air stripping column increase aeration reduce suspended solids
Low pH (less than 6.0)	add alkaline buffers (calcium hydroxide, potassium hydroxide) reduce feeding rate check ammonia and nitrite concentrations
High ammonia (above 0.5 ppm as un-ionized)	exchange 20% of system water reduce feeding rate check biolfilter, pH, alkalinity, hardness, and dissolved oxygen
High nitrite (above 0.5 ppm)	exchange 20% of system water reduce feeding rate check biofilter, pH, alkalinity, hardness, and dissolved oxygen
Low alkalinity	add alkaline buffers (KOH, CaOH)
Low hardness	add calcium carbonate (CaCO₃)