August 18, 2014
Every aquaponics practitioner shares the same fear: that one morning all the fish will be floating on the water’s surface. Although tilapia is one of the hardiest species raised in aquaculture, they still depend on well-managed water quality. Here are some lessons we have learned at the CLF Aquaponics Project.
Monitor Your Water Chemistry
Aquaponics is a living system that depends on a series of chemical and biological cycles and physical treatment of waste (i.e., filtration and sedimentation) to maintain healthy water. We regularly test for several parameters: pH (daily), ammonia, nitrite, nitrate, and alkalinity (weekly). With the help of bacteria, ammonia from the fish waste is converted into nitrite and then nitrate. Ammonia and nitrite are both highly toxic to fish, and we monitor these levels to ensure that they are within safe ranges. Generally, the system balances itself, but if these levels start to get too high, we perform a partial water change or cut back on our feeding rate. Nitrate can be toxic to fish as well, but only at very high levels. In aquaponics, nitrates serve as a fertilizer for the plants, and the plants reduce nitrate levels for the fish.
As ammonia is broken down into nitrate, the water pH gradually declines. We monitor pH daily, and add bases, such as dolomitic lime and potassium bicarbonate, to ensure that the pH stays within the safe range for tilapia, between 6 and 8. The nitrogen cycle described above is dependent on pH, and testing pH can also serve as a proxy for the overall health of the system.
The bases we add are a source of alkalinity, too. Higher alkalinity means the water has greater pH buffering capacity, and keeps pH stable. In the fall of 2012, our alkalinity declined to below 10 mg/L CaCO3 (generally we shoot for a minimum of 50 mg/L CaCO3), and we were unable to quickly correct the drop in pH. We experienced our first fish mortalities as a result.
Avoid Sudden Changes
Fish become stressed with any sudden shifts in conditions, even if the change is in the right direction. For example, if the water pH is at 6.0, and you raise it to pH 7.0 in a day, your fish and beneficial bacteria will experience a shock. pH is measured on a log-scale, so from pH 6 to pH 7 is actually a 10-fold change. Sudden temperature changes can also shock the system. Always make water additions gradually, and try to match the pH and temperature anytime you add a large volume of water to the system.
Think Before You Spray
Even if a product is “all natural,” do not assume it is safe for fish. Be very cautious when adding supplemental nutrients or pest control sprays to your system. Many all-natural pest control sprays, such as neem oil, can harm fish.
Like soil, healthy water in an aquaponics system requires more than just adequate concentrations of essential plant nutrients. Water in an aquaponics system is full of life, and maintaining the right conditions for optimum biological processes will pay off in healthy plant growth.