I've been reading about this for a couple of days, and it seem to be a very common symptom in aquaponics. The reasons I found so far, are between two:
- Iron deficiency.
- High pH level (+8), which prevents the absorption of iron.
Measuring the proper dose for your plants, that won't hurt your fish, is a bit of a mystery for me. However, you can always be extra careful and not introduce the added supplement in high amounts. Liquid iron can be found at pet/plant shops and online.