Good point Dentoid. The Yellow Wrasse is typically called peaceful, but there are always exceptions to the rule. I've always considered the type of territory a fish will inhabit when getting my fish, and tried to avoid mixing types that will inhabit the same territory. As an example, I can't add another goby as I'm sure my big shrimp goby would not appreciate another bottom dwelling goby trying to share his caverns under my rock. I even try to avoid different fish having the same coloration. The only "quarrels" I ever have in my tank are between my yellow diadema dottyback and my yellowtail damsel, and I think it's because the two share yellow coloration. No harm done, but the dottyback will infrequently chase the damsel. And my big goby will chase away any fish that comes too close to his favorite cavern - just an open mouthed bluff, but it works well! And my clowns will often hassle any fish that gets too close to their favorite corner of the tank, especially when they're protecting eggs (expected). I've always researched contemplated fish to see what type of territory they'll inhabit, and tried to avoid adding more than one that will want the same territory in the tank. Difficult to do in a nano tank, but I've been pretty successful with this approach as I've never had any real problems with any of my fish physically doing any harm to each other. With one exception - I started out with 2 Pajama Cardinals, and after over a year of one being constantly harrassed and at times physically damaged by the other, the least aggressive one finally died, probably from stress. I think I must have had 2 males, and the one just couldn't tolerate the other.