Yeah, be careful with the Flatworm Exit if you use it. I've heard it can wipe out tanks... not so much the medication itself per se, but rather the dying flatworms release toxins.
If you have a small flatworm population I wouldn't worry too much about it. Otherwise try to siphon as many as you can over the span of a few days. They are attracted to bright lights, so do it during the height of your lighting period. Once this is done, you can treat and just follow the instructions. Do a big water changes afterwards and run carbon and ozone if you have it (be sure to turn these off during the treatment).
I had to use about 1.5 times the recommended dose, and I treated about three times. There were still some stragglers but they died off within a month.
And obviously it is preferable not to use the medication if you can avoid it. I didn't have any fish or invert losses though. YMMV.
Some species of wrasse have been reported to eat flatworms, i.e. melanarus, leopard, etc. Some people recommend mandarin dragonettes but mine has never eaten any.
