This is a common misconseption!! Many critters like mantis shrimp and bristle worms are said to eat fish, coral. This is not true in most cases!! If you have a dying coral, then most likley will a bristle worm or mantis for example go in for a free meal. People see them eating their coral and emmidiatly blame them, even they had nothing to do with it. I know of people that have bristle worms 30-40 cm big in their tank. They are very hard to get out, and if you touch them you get millions of needle type things in you, hence the name bristle worms. Same thing for fish, they do nothing to them (in most cases, there have to me exceptions) and their diet is mostly scavange anyway, so they eat poo etc...
Be care full of urban myths in this hobby!