I added a hippo after I had a yellow in there, they were both about 4"-5" long. There was a lot of fighting in the biggining. I had to move the yellow into a 10G fuge for a couple of weeks to give the hippo a chance to catch his breath and get used to the tank. When I put the yellow back in there, he went back to harrasing the hippo. eventually they settled down and both are doing well now. I've heard people say use egg crate to devide a small section of the tank and place the more agressive guy (the established one ) there and let him get used to the fact that the new guy is swimming arround in the tank.