Just to add to the mandarin eating the flatworms. The dont many times or not enough to hurt the flatworms population. I personally have not had luck with them helping much. My yellow coris was better. I would guess they do pick at them but could think the tank would also need assistance by some other method , like siphoning.

Personally I would go with a melonarous wrasse. They are continuously on the hunt. I first got mine while having a pest problem with brissle worms. The coris wrasse is a good one but I prefer the above