none.
edit- I'll add a little info as to why imo it's none. They eat invertebrates.. they LOVE clean up crews. CUC is a yummy meal to them. I've seen them devour crabs like their made of candy. They also poop ALOT. So there we go, they poop alot + they eat the clean up crew.. who's gonna clean it up? A reef tank can't be a reef tank if you can't have anything in a shell. No snails, No crabs, etc. I've never seen them go after corals and I havn't looked that up.