Hi! I have a 55 gallon freshwater fish tank. It is stocked mostly with Platy's, guppies and tetras along with 2 swordtails and 2 Gourami's - 1 a dwarf red and 1 a larger gold. The swordtail is attacking the dwarf to the point where he has a large white ulcer/sore on the side of his body. It is not attacking anything besides the dwarf. Is there some way to stop this behaviour before he eventually kills the fish? The other swordtail in the tank does not attack anyone else. The dwarf gourami has only been in the tank for about 10 days. The larger one has been in the tank for about 4 years. Could it be just the new guy getting picked on?