Any fish you put in are going to have a hard time controlling a population that big. There are many fish that have been said to eat flatworms, but none that will consistently do it. Many people have mandarins (and other dragonettes) that won't touch the things. The Pseudocheilinus, Halichoeres, and Coris wrasses will sometimes do it. I've also heard a few people mention sunrise dottybacks helping out.
But, like I said, none of those are close to guaranteed to do anything about flatworms. Even if you do have some success, in a tank that size you're likely to need to syphon a lot to give the fish a chance.
Dave