I curently have a niger trigger with 2 volitans and 2 damsels in a 60. Doing great. The niger will be fine in almost all tank sizes from 20 on up. The niger needs alot of rcks and creavices to hide and roam
if your gonna put in live rock so he has places to hide there will be no room for the fish in a 20 gal ( personal opion ) a 20 gal is just as bad as putting him in a fish bowl