Morenicol Lernex Pro? It gets rid of nomatodes, gill flukes, tapeworms etc. after proper healing procedure. Which takes around 20-30 days. With multiple water changes and adding certain amount of this stuff to your tank, at the proper water temp.What does this stuff do to the fish?
The only way to know if you have any parasites is to look under microscope when your fish start to behave funny, or start to look worse.
Luckily - if you don't toss fish without quarantine, random plants, freshwater life/frozen foods (only saltwater kind like krill etc.), and use separate stuff (buckets etc.) if you have many tanks - you will not get any parasites once they are gone (or if you were lucky enough to get fish without them).
Besides that - besides healing - it does nothing "bad" to the fish.