DO NOT ADD A YELLOW if you allready have one. i did that and he all most died, so i took him out and bought two more bigger yellows. then i put all three in at the same time and the old yellow did'nt know what to do. picking on each other for a few hours then every one settled down. that was five months ago and all are fine. last month i added a large powder blue with no problem. now i have 4 yellows. 1 purple and 1 large powder blue. all getting along.