Nutrient supplied to plants in Aquaponics are from the fishes and the fish food dissolved. These sources of nutrient lack Iron, in Aquaponics Iron deficiencies result in yellowing of leaves.
There are commercial products that address this and there also some methods that are practice of old.
Chelated Iron is one common commercial product use, it is added to water via grow beds every about 2 ~ 3 weeks or so, this will cure plants of the Yellowing Leaves symptoms.
Blood meal also a source of Iron can be added to provide Iron to plants.
Old Farmers practice rusty nails and scrap metals in a bucket of water, and pouring it near tree trunk that have yellowing leaves interest me.
Does it work, those old timer may not heard of Chelated Iron, but they sure do have a lot of nails and scraps.
To practice what those farmers did, I collected scrap iron from around my back yard and put it in the tank.
At first I put it in the filter, but recently I found some long iron rod, those that for construction work they are too long for the filter so I use PVC water pipe to hold them.
What’s needed is a PVC pipes big and long enough for the job and 2 end caps. Drill holes along the pipe and place iron rod inside, capped the pipe and throw it in.
You can attach a string to it for easy retrieval or just without.
Photo shows the PVC pipe and Iron inside and this one with it in the water.