I have a powder blue tang that didn't take too kindly to his introduction. Chased him into the substrate , the Wrasse's natural behavior on display. I am more inclined to believe that he went up and over the overflow.The next question is, "what will you do to prevent this next time?" I assume it got down through slots at the top? I've had an anemone decide to walk to the slots, get stuck, then I had two anemones. I've thought about covering the slots with screen, but I haven't done anything ...