I have heard of these methods of success:
-mandarin goby...I personally had one. It ate the flatworms, but was also eating frozen brine shrimp as a back up when the flatworms were gone. I still have it and it is almost 18 months old.
-a velvet nudibranch...they work very well.....here are the stats on these guys via liveaquaria....I can order them, but they need to be picked up same day because I have no flatworms for them to eat!!
The Blue Velvet Nudibranch, also known as the Head Shield Sea Slug, is black in color with blue lines around the outside of its body and down the middle of its head. It has a hammer shaped head and two appendages at the rear, mimicking a flatworm.
This specialized eater will consume flatworms in the aquarium. It sucks up the flatworms using a tube-like proboscis and makes a great natural alternative for flatworm control. However, it must be provided with a steady diet of flatworms in order to thrive. In addition, it is sensitive to rapid changes in water conditions and does not have a long life expectancy. Because of this, only expert aquarists should attempt to maintain this species.
Spawning occurs regularly when more than one of this species is maintained together. Unfortunately, there has not been a good success rate reported in raising the larvae. The Blue Velvet Nudibranch should be acclimated using the Drip Method. It can be harmed by pump intakes and drains, so care should be taken to screen these off. It is sensitive to high levels of nitrate and will not tolerate copper-based medications