I have flatworms in my tank too. I recently used Flatworm Exit and it did not harm anything at all!! I just followed the directions EXACTLY!! As soon as I put in the recommended dose, within 3 minutes or so I started to see the flatworms dieing!! It was great!! But man.....you don't realize how many you have until you put the Flatworm Exit in and they start crawling out of everywhere!! There had to be hundreds if not thousands in my 180!! As soon as they started dieing I added the 4 pounds of carbon that the instructions told me to add. Everything was fine....including all my corals-SPS, LPS, etc....and all my fish!! Didn't even lose a shrimp!! And I didn't do a water change for about a week after!!
It didn't kill all the flatworms, so I am planning on another dose soon!!
My suggestion is to go and use it.....just have enough aged, salt water ready for a water change in the case you see any of your fish in distress. But I say use it.