Some new aquaponic units experience iron deficiencies in the first two to three months of growing as iron is important during the early stages of plant growth and is not abundant in fish feed. Thus, it may be necessary to initially add chelated iron (soluble iron in powder form) to the unit to meet the requirements for plants. The recommendation is to add 1 to 2 mg per litre for the first three months of starting a unit, and again when iron deficiencies are present. Chelated iron can be bought from agricultural suppliers in powder form. Iron can also be supplemented by using aquaponics-safe organic fertilizers such as compost or seaweed tea, as iron is aboundant in both

When iron (Fe) is deficient, plants will produce less chlorophyll in their new shoots. The rich leaf green fades, and the young plant parts assume a yellow to white color (chlorosis). Iron deficiency can be avoided by choosing appropriate soil for the growing conditions (avoid growing acid loving plants on lime soils), or by adding well-rotted manure or compost. You could add some more iron in a chelated form, for examlpe FeEDTA, but other chelators are better at higher pH values.

Please check this out -

https://university.upstartfarmers.com/blog/the-beginners-guide-to-iron-in-aquaponics