So that one fish may be fighting with the only other fish in the tank 'cause he's lonely? According to a recent study, certain species of fishies fight more and act less like themselves when they have few friends along for the ride swim.

The research examined a 2.6-gallon tank filled with either one fish, two fish, five fish, or 10 fish. Of all the pairings, the group of 10 was the "least aggressive, least likely to dart, and most likely to form schools and behave as they do in the wild." Even though the scientists only looked at neon tetras and white cloud mountain minnows, if you've ever owned fish, how many did you keep in a tank at a time?

Source: Discovery News