I had the yellow and the Hippo, and when I tore down my tank for fumigation, I added a purple to the hotel tank. None of them had a territory yet, so the purple and yellow got along fine. to be safe, since you will be adding a purple to the yellows territory, put the purple in a breeders box/breeders net for a week or so to try and let them get used to each other... Clifford did that and they get along fine now for him as well.