In Aquaponics the perfect balance would be to feed the fish to have the fish feed and source water provide all the necessary nutrients for the bio-filtration and the plants, however in the real world it can sometimes be difficult to achieve such a perfect balance. When system water pH is high or the source water doesn’t provide any iron, it is sometimes necessary to supplement things like Iron. When supplementing Iron it is often beneficial to use chelated iron because it remains more available to the plants even at higher pH. The type of chelated Iron can also affect the availability of the iron to the plants. EDTA Iron is the least beneficial to the plants with DTPA providing better availability and EDDHA providing the best crop yield, however at the greatest cost.
In Aquaponics the plants only need about 2-3 ppm of Iron available in the water or planting media. It is not desirable to over do the amount of iron. For backyard growing I would usually recommend adding enough to keep iron deficiency at bay by keeping an eye on the most iron deficiency sensitive plants for the first signs of when you need to add iron and learn about how much to dose every few weeks to keep the deficiency from showing up. If one wishes to get more specific than that, Iron test kits can be found for under $100. Commercial scale operations will need to invest in the water chemistry testing supplies to be able to track iron levels and dose accordingly.
Iron levels and dosing schedules are not something that can be dictated by a book since the source water, fish feed, crops grown and even the system type/media will affect the water chemistry and iron levels.
Iron deficiency show up as yellowing of the leaves while the veins remain green. Signs of Iron deficiency show up on newer leaves first. (If newer leaves are green while the older leaves are showing chlorosis then it is probably not Iron deficiency.