I came in from class today and my lima looked funny, his colors/fins etc looked fine, however it looks like his back end is sagging...i dont have a camera, but everything about him looks fine except his spine looks like theres a slight arc. he also seems a little slower and not as active, i tapped the glass once and he darted around normal (dont usualy tap the glass), i change the water 15-20% a week, and went to feed him today and decided to destroy a chunk of shrimp before eating it, i dont know what to think..anything i should do?