Iron is one of the nutrients that need to be monitored regularly in aquaponics. We test the iron levels in our aquaponics installation every 2-3 weeks using a standard aquarium kit.
The ideal iron levels for our aquaponics setup, which uses freshwater catfish, are somewhat higher than those of a typical aquarium. Our ideal iron level is somewhere between 5.0 and 8.0 ppm.
The combined volume of our five fish tanks and sump tank is approximately 4800 liters, which calls for about 2mg of iron per liter. To maintain the correct concentration of iron, every 2-3 weeks we dissolve 137g of Fe-DPTA chelate. We had experimented with other iron chelates in the past, but Fe-DPTA has proven to work for us in addition to being recommended for most systems.
We dissolve 10g at a time in a liter of water, and distribute it among our plants, typically directly into the grow bed siphons, rather than adding it directly to the tanks. By regularly supplementing iron to our entire system, we can maintain a relatively constant level of iron and prevent our plants and fish from ever developing a deficiency.