Ok I've had my tank established for about 2 months maybe almost 3. I started out with 1 Gourami, 1 Bala Shark, 2 Mollies, 5 Red Eye Tertas, and 2 Black Skirt Tetras, and 2 common plecos, and they all got along just fine no picking at each other or nothing, about 3 weeks ago I added 2 lace gouramis, 1 opaline, 1 red tail black shark, 2 tinfoil barbs, 2 silver dollras, and 1 more common pleco. Since then they all have gotten along fine, no picking or nothing, they just swam in their groups or solo. Well tonight I come home and the red tail shark is sucking on the side of the orignal gourami. What is causing this? What should I do, they all look fine, and healthy, also he has started going to other fish and do the same.