Yellow tail blue damsels have a reputation of the most peaceful of the damsels, but, like all damsels, can have a mean streak. I had a bad experience with them (mean SOB) and many others have as well, while others have had nothing but luck with them fitting into a tank. So chances are you will be fine with adding one, but there is a small chance that he will be a mean
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and torment the others (mine had an ocellaris and a lemonpeel, each 10x his size, pinned in one corner of the tank) and if that happens, good luck catching him...