They'd probably eat cattails. My pigs really loved them and I've read cows eat them. Or water cress. Or possibly duck weed, which can also feed fish.
One thing that confuses me about aquaponics systems is that when I raised tropical fish the fry would give off hormones that would suppress their growth. We did 90% water changes daily to get the best growth out of them. If you recycle the water so much, is there anyone looking at whether the growth-suppressing hormones build up in the water, or which plants best break them down?
I've read of crayfish aquaponics systems, tho, and the marmorkrebs (peaceful, self-cloning crayfish) didn't seem to have as much growth-suppression as the fish fry did. They'll also live happily on plant food so you're not killing 10 lbs of wild fish to make the feed to grow 1 lb of "farmed" fish. When you do feed meats to marmorkrebs, they could handle more saturated fat than fish could. In cold water the saturated fat goes too solid for the fish and makes them unhealthy, which means most factory-farmed
land animal-based proteins make very poor fish food.