Superficially, focus on driving down net waste in the system.

Take the nutrient profile of tilapia waste. Compare that with the nutrient demands of plants to find candidates. Then look for plants where the majority of the biomass can be converted into food that the tilapia can digest (i.e. edible roots, stalks, leaves; the more the fish can eat the less energy you take out of the system).

IIRC at least one of the plant choices had a tuber.

IIRC there is also something about the efficiency of waste->biomass conversion for the plants at different stages of development; figuring out when the ideal time to harvest is to minimize energy loss.