Article written by: Jason Danaher Ph.D. 

Aquaponic systems require regular monitoring of water quality on a weekly basis to ensure good fish and plant health. Important water quality parameters to measure and monitor include temperature, oxygen, pH, ammonia, nitrite, nitrate, total alkalinity, total hardness, and iron. Most of these parameters can be quickly and accurately quantified with a water quality test kit or the combination of a water quality test kit, specific handheld meters, or a benchtop meter. We will briefly discuss important water quality parameters and provide a general range to consider initially for optimal fish and plant health. With experience the aquaponic farmer can tweak parameters at their specific location.

Temperature

The majority of aquatic organisms grown in aquaculture are poikilothermic. Their body temperature is similar to and dependent on the surrounding water temperature. Temperature directly affects their metabolic rate, their locomotion, and feeding behavior. Temperature influences the desire of the fish to obtain food, how they process food through digestion, and their respiration rate. Temperature also affects other water quality parameters important to fish health. Generally cold water fish prefer temperatures in the range of 40 to 55F; cool water fish prefer temperatures 55 to 70F; and warm water fish prefer a temperature range of 70 to 85F.

Plant roots are also very sensitive to water temperature. The optimal water temperature for hydroponics will depend on the type of plants you are growing. Most plants generally prefer a water temperature between 65 to 80F.

Oxygen

Fish, like all animals, need oxygen to survive. They use oxygen for essential functions like metabolism, growth, and reproduction. Sensitivity to low levels of dissolved oxygen is species specific. In general, a minimum oxygen concentration of 5.0 mg/L is recommended for optimum fish health. Many species of fish are distressed when oxygen concentration falls in the range of 2.0 mg/L to 4.0 mg/L for even short periods of time. Depending on the species, fish mortality can occur at concentrations less than 2.0 mg/L. Generally, cold water fish are more sensitive to low oxygen concentrations (less than 5.0 mg/L) than warm water fish.

Maintaining proper levels of dissolved oxygen is also crucial for hydroponic plant roots. Plant roots are living tissue and serve a crucial role in nutrient uptake and overall plant growth. One of the primary reasons why dissolved oxygen is essential in hydroponic setups is its direct impact on plant health and nutrient uptake. Hydroponic plants prefer oxygen levels in the root zone equal to or greater than 5.0 mg/L.

pH

The pH scale ranges from 0 to 14. The pH characterizes a solution as acidic or alkaline. A solution is acidic if the pH is below 7.0 and alkaline if the pH is above 7.0. A solution with a pH of 7.0 would be described as neutral. Fish cannot survive in waters below pH 4.0 and above pH 11.0 for long periods. The optimum pH for fish is between 6.5 and 9.0 and fortunately this range is often maintained within an aquaculture system. The pH level, alone, rarely is the direct result of fish mortality; however, the pH can have an important effect on other water quality parameters and make them more toxic for fish.

Maintaining proper pH level is also crucial for hydroponic plant roots. One of the primary reasons why pH is essential in hydroponic setups is its direct impact on nutrient uptake. Nutrients may be present in the water, but the chemical form and ability of plant roots to transfer nutrients from the solution into plant tissue is dependent on maintaining optimal pH levels. Most hydroponic plants prefer a pH range of 5.5 to 6.5.

With an aquaponic system the operator needs to take into account desired pH levels for the fish and plants; therefore, optimal pH range for an aquaponic system should be maintained between 6.0 and 7.0.

Nitrogenous Wastes

The protein in feed is composed of amino acids. Nitrogen is a main element in amino acids and when metabolized by the fish the nitrogen is released through the gills in the form of total ammonia nitrogen (TAN). The TAN is then metabolized by beneficial bacteria and transformed into nitrite (NO₂ -). The nitrite is then transformed into nitrate (NO₃-) by beneficial bacteria, also. These forms of nitrogen and the nitrification process are discussed more below and are important water quality parameters to monitor in aquaponic systems.

Nitrogen cycling in an aquaponic system is the process where beneficial species of bacteria establish themselves in a new system over time. These bacteria can live on the surface of the tank and in the biofilter. Through water quality monitoring an aquaponic operator can determine whether a system has completed its nitrification cycling period. Nitrification or the breakdown of toxic ammonia into nitrite and finally nitrate is essential for a healthy aquaponic system. Figure 1 below shows the theoretical steps of nitrification in the aquaponic system. When cycling, consider that at least 4 to 6 weeks is needed to experience nitrification before growing fish and plants in a well-functioning environment.

Figure 1. Chart showing the theoretical nitrification process over time at the start-up of an aquaponic system.

Total Ammonia Nitrogen

The total ammonia nitrogen (TAN) consists of ammonium (NH₄+) and ammonia (NH₃). The ammonium (i.e. ionized ammonia) is generally categorized as non-toxic; however, ammonia (i.e. un-ionized ammonia) is very toxic to fish. These two forms of TAN and their proportion present in water are dependent on pH and water temperature. A greater proportion of toxic, un-ionized ammonia exists when the temperature of the water is high and the pH of the water is above 7.0 (i.e. alkaline). The ratio of ammonia:ammonium decreases when pH and temperature decrease. The pH has a greater effect on ammonia toxicity than temperature. The combination of high pH and elevated total ammonia nitrogen concentration can lead to elevated ammonia concentrations potentially harmful to fish. To measure the concentration of un-ionized ammonia in a sample follow the steps provided below:

1. First, measure the pH and temperature of your water sample.
2. Second, measure the TAN concentration using the appropriate method available in your water quality test kit.
3. Reference the table below adapted from Emerson, K., R.C. Russo, R.E. Lund, and R.V. Thurston. 1975. Aqueous ammonia equilibrium calculations: effect of pH and temperature. Journal of the Fisheries Research Board of Canada. 32:2379-2383.

          4. Determine the percentage of toxic un-ionized ammonia in the water sample using the table, sample pH, and sample temperature. 

          5. Multiply your TAN concentration measured from the test kit result by the percent from the table to calculate the concentration in ppm (mg/L) of toxic (un-ionized) ammonia present in the water sample.

Safe levels of TAN generally range from 0.25 to 2.0 mg/L for most fish grown in aquaculture; however, toxic un-ionized ammonia is capable of killing fish at as low as 0.1 mg/L for coldwater and 0.5 mg/L for warmwater species.

Plants can take up ammonium directly, but the method of absorption depends on factors such as plant species, temperature, growth stage, and pH in the root zone. Most hydroponic plants do not prefer ammonium as a source of nitrogen for plant growth and generally do not take it up directly in an aquaponic system. In fact, ammonium is typically taken up through plant leaves which are not in direct contact with the water. Therefore, oxidation of TAN through biological filtration (i.e. bacteria) into non-toxic forms is essential for plant roots to extract nitrogen for growth.

Nitrite

In aquaculture systems, the nitrite (NO₂ -) is a product of the biological transformation of TAN as it is metabolized by Nitrosomonas spp. bacteria. Nitrite is highly toxic to fish and can interfere with oxygen transport in the blood causing fish to suffocate. Nitrite toxicity can occur when nitrite levels rise and dissolved oxygen levels decrease to concentrations less than 3.0 mg/L.

Safe levels of nitrite generally range from 0.20 to 1.0 mg/L for most fish grown in aquaculture. Hydroponic plants cannot take nitrite up directly. Therefore, oxidation of nitrite into non-toxic forms through biological filtration (i.e. bacteria) is essential.

Nitrate

Nitrate (NO₃ -) is the product of the biological transformation of nitrite as it is metabolized by Nitrobacter spp. bacteria. Nitrate is generally considered non-toxic to fish.

Safe levels of nitrate generally range from 5.0 to 150 mg/L for most fish grown in aquaculture. Hydroponic plants prefer nitrate as the source of nitrogen for growth in an aquaponic system and will take it up through their root system.

Total Alkalinity

Total alkalinity is an important parameter in aquaponic systems. Water with higher alkalinity (greater than 50 mg/L) is better buffered against pH change than water with lower alkalinity (less than 25 mg/L). Alkalinity is frequently measured in aquaponic systems and test results are reported as calcium carbonate (CaCO₃ ) equivalent. The total alkalinity concentration may quickly decrease as a result of biological and chemical processes occurring in the system. Two common sources of carbonate are agricultural limestone (CaCO₃ ) and dolomitic limestone [CaMg(CO₃ )₂ ]. Both are safe for aquaponic systems because they will not raise the pH quickly like forms of burnt limestone; however, neither dissolves quickly into solution. Both also contain low levels of sodium. Sodium bicarbonate (NaHCO₃ ) should be avoided in freshwater aquaponics as frequent additions will result in increased dissolved sodium levels that will negatively affect plant production over time.

While important to hydroponic plants, the total alkalinity is not a nutrient directly taken up by roots. Total alkalinity has an important indirect effect on plant growth by maintaining a consistent pH of the nutrient solution and allowing available macro- and micronutrient uptake by the roots.

Optimal ranges for total alkalinity are generally 25 to 100 mg/L, measured as CaCO3.

Total Hardness

Total hardness is often a measure of the elements magnesium and calcium dissolved in the water. Both of these elements play an important function in biological and chemical processes for the fish and plants within the aquaponic production system. Two common sources of calcium and magnesium are agricultural limestone and dolomitic limestone. Agricultural lime is labeled ‘calcitic’ because the product is comprised mainly of calcium carbonate. Generally, dolomitic limestone is comprised of approximately 50% calcium carbonate and 40% magnesium carbonate. Agricultural lime is often sufficient to increase total hardness concentration.

Optimal range for total harness is generally 20 to 75 mg/L. Like total alkalinity the total hardness is reported as CaCO₃ . Since both are reported as CaCO₃ they are often thought to be the same; however, total alkalinity quantifies how certain anions act according to changing pH, while total hardness focuses on specific cation concentrations at the time of sampling.

Iron

Aquaponic systems are often iron deficient due to low amounts of iron in commercial fish feeds. In addition, iron reacts with oxygen in the water and becomes chemically bound and unavailable for plant uptake. Iron is one of the essential micronutrients for plant development in aquaponic systems. Iron is an essential element for photosynthesis and many other cellular functions for plants. Therefore, iron needs to be supplemented to ensure optimal plant growth and health. Usually iron is supplemented with a chelated form. Chelated iron is treated to allow the iron to remain in solution and available for plant uptake.

There are several chelated forms of iron available for plant production, but not all are applicable for aquaponics because of the pH requirements for aquaponic systems (i.e. 6.0 to 7.0). The first chelated form of iron is Ethylenediaminetetraacetic acid (EDTA) and strongly holds iron in solution up to pH 6.0. By pH 6.5, almost one-half the EDTA iron has precipitated and by pH 7.0 almost none of the EDTA iron is available to plants. The second chelated form is Diethylenetriaminepentaacetic acid (DTPA) and is an excellent iron source up to pH 7.0; however, over 50% of the iron is precipitated and unavailable by pH 8.0. The third is Ethylenediamine di-o-hydroxyphenylacetic acid (EDDHA) which is the strongest chelate and maintains iron availability above pH 9.0. Oftentimes DTPA is preferred for aquaponics because it is the most cost effective for the desired pH range.

Adding chelated iron requires close management and regular addition for aquaponic systems. Optimal levels of iron generally range from 0.5 to 2.0 mg/L for most plants grown aquaponically.

In conclusion, a recommended concentration range exists for each water quality parameter in an aquaponic system and simple to advanced technologies are available to monitor each water parameter on a daily, weekly, or monthly basis. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 or email info@aquaticed.com to discuss water quality in your aquaponic system and various technologies available to monitor each parameter.