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| Carolina Tips | J A N U A R Y 1 9 9 7 |
Making a Splash in the Classroom
Nitrification Ammonia builds up in aquaculture systems as a by-product of fish metabolism. Protein in fish food is converted to both fish flesh and ammonia. The two forms that occur in water are un-ionized ammonia (NH3) and ammonium (NH4+). Total ammonia (NH4+ plus NH4+) in mg/L or ppm is most commonly measured with a test kit. Un-ionized ammonia must be mathematically calculated from a chart, based on water temperature, pH, and ammonia levels. High temperatures and high pH levels cause lethal concentrations of un-ionized ammonia, which is probably responsible for more fish kills in the classroom than anything else. At a constant temperature of 77 °F, un-ionized ammonia is 10 times stronger at a pH level of 8 than at 7, and 25 times stronger at a pH level of 8.5 versus 7. It is easy to eliminate the possibility of disaster if ammonia and its relationship to temperature and pH are understood. Ammonia is normally controlled in the system by Nitrosomonas bacteria, which oxidize it to nitrite. Higher levels of ammonia usually occur at the start-up of the system, again when the system approaches its maximum capacity, or when something has caused a setback to the bacteria. If necessary, a water exchange reduces ammonia levels. Remember to detoxify chlorine and chloramines from tap water before introducing it into the system. Nitrite (NO2) is the second leading cause of fish mortality in the classroom. Nitrite is the intermediate product of biological nitrification resulting from the oxidation of ammonia by the Nitrosomonas bacteria. Nitrite should be measured for most species with a test kit and kept below 0.5 mg/L or ppm. A 0.5% solution of noniodized salt can be added to the culture water to help reduce the effect of nitrite toxicity. However, this salt solution adversely affects hydroponic efforts if the water is shared with them. Generally, water exchange is the best method of nitrite reduction. Nitrite levels peak during the initial start-up, during periods of overfeeding, and again when the system nears its maximum carrying capacity. Nitrate (NO3) is the end product of nitrification and is normally not toxic to food fish even in high concentrations; levels as high as 200 mg/L are not uncommon in well-established systems. It is measured with a test kit. Nitrate is naturally removed from the water because it is a food source for algae and other plants. It is also removed through the water exchange that takes place during clarifier cleaning. The nitrification process is so critical that a short review is in order. Fingerlings (small fish) are stocked into the system and feeding begins. A percentage of the food is metabolized into ammonia. Nitrosomonas bacteria oxidize this ammonia into nitrite. Nitrobacter bacteria oxidize nitrite into an end product of nitrate, which is removed through plant uptake or water exchange. These bacteria are usually present in water everywhere; however, in new systems stocking them is recommended. Most packages or systems come complete with a bottle of liquid bacteria but, if not, they are available from a number of sources. |
pH Potential of hydrogen (pH) is a measure of the acidity or alkalinity of the water. It is measured on a pH scale from 014, with 0 being very acidic, 7 being neutral, and 14 being extremely alkaline. It is important to monitor pH because it influences how well the bacteria convert ammonia and how efficiently the gills of the fish operate. Along with temperature, pH helps to determine the toxicity of ammonia (un-ionized). A simple test kit or a hand-held meter can be used to test pH. In a classroom system, the pH should be maintained between 6.8 and 8.4, with a level of 7.0 to 7.3 being optimum for most species of fish. The addition of sodium bicarbonate (baking soda) to the water can increase pH. The introduction of muriatic acid can decrease it. (Always use caution when working with muriatic acid.) Levels of pH should never be adjusted more than one unit per 24 hours. Alkalinity Alkalinity is a measure of the quantity of compounds that shift the pH to the alkaline side of neutrality (above 7), and is mostly influenced by bicarbonates, carbonates, and hydroxides. Alkalinity is important because it buffers pH changes that occur naturally during photosynthetic cycles, water exchanges, or acid introduction (to lower pH). It also acts as a food source for Nitrosomonas and Nitrobacter bacteria. Levels of alkalinity should be maintained higher than 50 mg/L at all times, with levels up to 200 mg/L being desirable. Alkalinity should be measured with a test kit and can be raised with the addition of sodium bicarbonate or calcium carbonate. Raising the alkalinity almost always raises the pH. The pH should always be monitored during alkalinity increases, as a high pH increases the toxicity of un-ionized ammonia. Temperature Temperature influences many factors of the system. A cold temperature decreases fish respiration, decreases the conversion of ammonia, and decreases the toxicity of un-ionized ammonia. Depending upon the species, cold water can increase the potential for disease and even lead to death. Warm water increases fish respiration, increases the toxicity of un-ionized ammonia, and increases the efficiency of the biofilter. Warm water holds less oxygen than cold water. Temperature is best monitored safely with a nonmercury-filled thermometer. The optimum water temperature for aquaculture varies greatly with the species of fish. Trout and salmon generally prefer temperatures below 68 °F, while hybrid striped bass, catfish, and bluegill prefer the 80 °F range. Tilapia do best at temperatures near 90 °F. I recommend that you grow a species that does best with the normal controlled temperature of the classroom. Keep in mind that it is usually more economical to heat the water than to chill it. A wide variety of heaters and chillers is available, if temperature manipulation is desired. |
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