I had one and it was "devilish"! At first, it was black. Then when it settled in, it was a very pretty blue. It went after anything slightly bigger, the same size or smaller. I had to get rid of it. However, I guess it will depend on the size tank and the amount of live rock it has for hiding. I have a blue sapphire damsel and it is totally chilled out. It doesn't mess with any of the tank mates, not even the maroon clown. I have a cleaner wrasse that will hang out on all the fish and the sapphire damsel doesn't mind it. Liveaquaria says it has a peaceful temperment, but does warn that it could get aggressive as it ages or if you add multiple damsels. Although, charlieweaver had several together and they didn't fight. When I saw them in his tank they were schooling.