Flatworm Exit!!!!! By far the quickest and most effective. Especially if you have a large outbreak. I've tried wrasse's but if you feed your tank they will eat the food and not the flatworms. Be sure to turn off the skimmer and use charcoal.
I put one wrasse in a 300 system and all flatworms were gone shortly there after - never ran flatworm exit. These were the small clear ones, not the large dark brown ones...
Flatworm Exit made by Sailfert is the best. Quick and easy like the others say. Just don't use the BlueLife brand i have used that before and the flatworms came back after multiple treatments. I have never had a problem with Sailfert's Flatworm Exit but you want to follow the directions really good and don't skip a step. Especially the sucking them out part. Use airline tubing, that works best.