My tanks:
Tank1 - 3 Rays, 1 AUL, 3 Arows, 22 Dats, Tigrinus, 1 Giant Fei Feng (RIP), 3 19-20" Endli Bichirs and a few other fish. Most of these fish are from 10-20". Teh rays are 12-14" each
Tank 2 - Armatus, Rays, Juruense Cat, P-Bass, More Fei Fengs, African Aro, cuban gar, tropical gar
It all depends on the fish and how much you can dedicate to feed them..to feed the rays, tig and aul on the big tank, I use 3/4" clear acrylic tube. The rays are used to come up and grab food from under the tube.
stan