If you're going to dose your exisiting tank with everything in it then I 2nd or I guess 3rd the above, remove as many of th e flatworms as you can before dosing. When flatworms die they produce a toxin which in large doses is harmful to fish. The flatworm exit does work but cautious how you use it. When I had flatworms I removed a rock at a time and dipped it. It worked like a charm and I haven't had any issues since. I've also read that a coris wrasse will eat them but this is never a guaranteed thing. Good luck!