I just recently setup up a 28 gallon and tied it into my existing 55 gallon system with the intention of using it for breeding onyx clownfish. I purchased 3 clowns (1 onyx, 1 black, and 1 orange ocellaris) about 3-4 months ago. They have been doing great, and shown absolutely no aggression toward each other. The other day I purchased a second onyx clown, put both onyx clowns into the 28g together, and closely observed for about 30 minutes. The 2 onyx clowns were constantly aggressive towards eachother for the full 30 minutes until I pulled the new one out and put it into my main tank. I have tried multiple strategies including putting all 4 clowns together in the 28g, but the 2 onyx clowns continue to be aggressive without showing any submissive signs (ie: body vibrations, etc.). The only idea I have left is to purchase another onyx and put the 3 onyx clowns together, but as they are rather pricey, I would prefer not to. Any help would be greatly appreciated!