Planaria and use potassium chloride as a dip. You can use 2 teaspoons in one gallon of tank water for about 30 minutes. You can make the dip stronger (1-2 tablespoons per gallon) for a shorter time, 10 minutes or so. There are a few threads out there with recipes, times, etc. If you don't want to use potassium chloride, the pick up some Polyp Lab Reef Primer, it's essentially potassium salts. It's probably one of the best dips you can buy for flatworms.