I love these fish! Beautiful, active and relatively easy to keep. Not at all picky about what you feed them . Peaceful, but will eat smaller fish if given the opportunity. I find that when the are younger they prefer to be in small groups like many others have stated. Very fun watching them swim around all over the place. When they get older, the seem to do fine alone. I've had mine for about 4 years and they no longer seem to enjoy each others company, and don't really bother with each other. They aren't agressive , just rather not be bothered. As for tank size, I would say 55 and above would be best as they get pretty big and are very active. As for tank mates, anything that they can't eat, they won't bother.