I've heard different variations on "feed your fish only the amount that they can consume in about 1 minute", and these make sense in terms of the amount of biomass you want to introduce into the tank.
I've never seen any studies on the long term effects of overfeeding one's fish. I would assume that they would become sluggish, but are there any other long term side effects on their development?
Well, not exactly about "pets", but there is at least on study about overfeeding fish.
This article from Aquaculture International, published by Springer, studied the effects of overfeeding Tench.
Some conclusions related to the nutritional value the fish will have (like it´ll have more fat deposits and smaller minerals content).
But some of the findings could be applied to pet fish, too:
incidence of uneaten food (that will cause some unbalance in your aquarium, increasing ammonia content, etc)
deformities of the caudal penduncle
retarded growth
and although no mortality was observed, it´s perhaps because the study was done in 70 days, but the high rate of deformities might lead to deaths in the future.
