Flatworm Exit won't harm corals or fish. Just follow the instructions and try to stay away from googling flatworms. You'll see all kinds of horror stories and potentially freak out. Just remember, the most toxic part of the process is when the flatworms die, they release a toxin. If you go ahead and treat for them be fore they are covering every single square inch of your aquarium, you'll be fine. Some folks nuke their tanks when they treat because their systems are literally covered with the things.