Originally Posted by sepulatian
Ren, it realy depends on how much rock you have in the 20 and what kind of damsels you want. I have had 2 yellow tails (supposed to be the calmest next to clowns and chromis) fight like hell in a 55. They also do not like new commers. Dominoes and four stripes are two of the most aggressive varieties. I, personally, would get like 3 blue chromis if anything. Damsels can be nasty. Just a heads up.
I'm guessing the two yellowtails fought with each other, which is why I suggested not getting two of the same kind. In the confines of an aquarium, damsels really don't like having someone else just like them around. Other than that, yellowtails are quite capable of getting along with other fish and other types of damsels. The yellowtail I have now is completely dominated by my green reef chromis (which kind of goes against the common belief that chromis are always peaceful). And four stripes aren't among the more aggressive; their three stripe cousins are. I've kept four stripes with clowns, psuedochromis and other damsels without any real problems.
Anyway, if you get 3 or 4 damsels that are all about the same size and tempermant, they should balance each other out. It might also be a good idea if you could put them all in at the same time, or at least start with whichever one is considered the least aggressive.