Six Line Wrasse
My 55g tank came with flatworms. Some suggested a Six Line Wrasse as a natural predator for these small rusty pests, but alas, mine didn't seem interested in them. However, it added beautiful color to the reef, and disturbs nothing. I purchased it when it was very small, and no-one could get over how large and fat it grew to be. This fish may be Reef-safe, but some report how overly aggressive it can be toward other tankmates. The Four Line Wrasse might be a better alternative.