This tank is going to be a major PITA. The tigers and polypterus prefer neutral to alkaline water. The angels and motoro need water on the acidic side. Also, the temp requirements are higher for the acidic fish. And, even by itself, a 50 is just wrong for any ray. And, not due to the ray's eventual size but, because the water quality will change too radically, for a ray, in a tank that size. Their diet and waste production simply make a 50 too impractical maintenance-wise.