2 issue ive heard. First one is a big one; Moving around. They pack a solid sting that *can* kill or injure corals. Sometimes they will sting them badly and other times not. Second Issue is the sting itself. Weak swimmer can become lunch. The potency of a maxi mini is more than a Bubble Tip nem, but far less than a Condy. I haven't had an issue with either but if a maxi mini stung a purple monster or red dragon to death I would be ticked. Also I dont have either of those lol.