Your tank is cartoonishly overstocked. Even if you have a dwarf angel and a small species of butterflyfish, two tangs and multiple clowns in a 50 gallon tank is a disaster waiting to happen. I can't recommend any invertebrates until we get this straightened out first as your water quality is in jeopardy at this point. Please post back with the specific species of fish that you have so we can help you pare down your livestock to a more reasonable level.
EDIT: Saw your fish list in your temperature question post. Assuming that you don't have maroon clowns (they get huge), you could keep a pair of any other clown species, the pelewensis butterfly, and a flame angel successfully in this tank. However, the tangs need to go. You're looking at a good 9" apiece on those tangs and they need far more swimming room than a 50g tank can provide. I don't mean to rain on your parade, but those fish are under a good bit of strain right now, and powder blue tangs are quite fragile to begin with. You need to get both tangs back to the store ASAP.