Shoaling fish (meaning, fish that live in large groups in nature) must be maintained in groups in an aquarium. Six is usually considered minimum for most shoaling fish, though there are exceptions (needing more than 6). When shoaling fish are kept in smaller numbers, aggression is often the result, even from otherwise "peaceful" species. Not long ago I posted a link to a recent study proving heightened aggression in shoaling fish when the group was less than 5-6.
The other issue is tank size; fish in tanks that do not provide enough space also will frequently show increased aggression, or aggression from again an otherwise peaceful species. Fish that like to swim, such as White Clouds, need space to do so, or they will feel "boxed in."
Temperature is another factor. You've mentioned that, so I won't say more, other than keeping cool water species in warmer tanks can cause stress, and aggression is one result of stress.