It is good that he is eating - but to be honest the stingrays are not the only risk.

Blood parrot cichlids, elephant nose, ghost knives, kissing gouramis, and rainbow sharks are ALL semi-aggressive fish. When put with a creature who is not exactly semi-aggressive but acts more out of filling its gut then defending territory, you are asking for trouble.

Just because the frog has been lucky so far, does not mean it will continue to do so. Fish increase in aggression as they age - you could just be waiting for the inevitable unfortunately.

The bloat could be indicating a stressed, unhappy, unhealthy frog.