I agree with you about chemical use in general, but our tanks are not natural by any means. We use a big glass box, synthetic salt, fake lights, equipment to filter the water, additives (chemicals) to balance out water chemistry, etc. You can't always fit nature's controls into your tank. ;)
Sometimes you have to make a choice. If a pest is going to take over the tank and kill things, is it worth it to not fight back? Flatworm Exit is very safe. If something dies because of it, it's because you had too many flatworms that died and released their toxins, not because of the Flatworm Exit.
There isn't a good way to naturally exterminate flatworms in our tanks. They multiply too quickly, and fish can't possibly keep up with them all by eating them.
And nature does present problems that it can't fix all the time. That's how natural extinctions happen.
The mandarins are the green fish, but they don't always eat flatworms. Sometimes they don't eat anything but one type of food, and starve to death in our tanks.
Coral banded shrimp are known to be aggressive towards fish regardless of the tank size. And I've never heard of them eating flatworms, although it's possible. As far as I know, you can't keep a pair of coral banded shrimp. They will fight.
