To identify the issue, we use University of Arizona's assessment charts that help us recognize the symptoms corresponding to each nutrient deficiency.
A lot more information about nutrient deficiency can be found on Montana State University website.
We are now trying to find fish and humans friendly remedy to each deficiency and testing how to use them.
When adding a product, always dilute it in some water and pour it into the decantation filter (or growbeds depending on the aquaponics setup and nutrient deficiency).
Iron: between 2 and 10 ppm
Librel IJzer FE-DP DTPA 7% 1kg/pak : https://webshop.karobv.nl/pd/BR150212151?searchText=FeDP%207%25