Can I treat my fish to too much food?

Solution 1:

If you really want the fish to live, I would suggest you buy the Acquarium VI at the Presidium Commons on Citadel for 25k credits. I've never heard of overfeeding causing the fish to die, but the consensus is that the way to go is feeding them after every mission you complete.

Solution 2:

According to some vague sources, fishes can be overfed. But to be honest, if you ignore the fish, they die. If you feed the fish, they die. If you have Kelly feed the fish, they die (That's in Mass Effect 2). Bioware just created the fishes to troll the players IMHO.

It seems that your misfortune with pets is a common problem in Mass Effect. I know few (read: no) people that raised their fishes to be healthy and happy.

In addition, a tip to keep them alive as long as possible: It seems that fishes survive longest if you feed them after every assignment or mission you complete.