I have 2 tiger barbs and 2 green tiger barbs in a 29g with 5 black skirts, and I discovered today that one of the tiger barbs is nipping one of the green barbs considerably. I feed them well and they are on a varied diet, and it's only 1 tiger barb nipping one of the green ones, at least I can't see any damage to the other green barb. the 2 green barbs are fine around the other tiger barb. The barb that is doing the nipping won't even let them near food if he sees them. what's causing this, and how can i stop it? will he kill the green barb??