actually, yes. first off, are they all males, all females or mix? how many of each do you have? fish are like dogs, there is a pecking order, and they will "fight" for position as top fish. my mollies can't be with any other fish but bottom dwellers, they can be pretty mean actually (never killed anything, just pests). if you have all males or all females, part of its to establish the pecking order, part is because, well, theyre restless, and need members of the opposite sex. if you have a mix, its both the pecking order and a right to mate (top fish male gets all the girls). i had two males and three females together, the one male always chased off the other male. eventually the females got in on the bullying (tail slapped him silly) and the one male spent most of his time hiding in the cave or in the plants. i've moved him to the fry tank where he is happy being the biggest fish (and i think the only male, looks like all my fry are female......). also, since you added them separately over time like that, part of it may be a territory thing, try moving your decor around a bit and see what happens.
