Firefish are a subgroup of the gobie family and like other gobies, like to be kept in pairs or schools. But since the sexes are virtual impossible to tell apart, I am not sure how you would know a pair unless you purchased them as a pair that a lfs has already done the "hard part" and either purchased them as a pair or let them pair off . I wouldnt suggest just purchasing two and hoping they are a pair. Either buy several or just buy one. I had purchased three at once and they have been doing great - no problems for the past year, though I do think that 2 have paired off and there is an odd one, they dont show any agression toward the "third wheel".