+1 on Cbau comment "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."

IF you use Flatworm Exit it is best to do it at night about a hour or two after lights out. Reason being is while the chemical isn't toxic to fish or corals the dead and dieing flat worms can potentially harm your fish. Make sure to not run your skimmer or and carbon for 24 hours then resume and do a 20% water change. I believe if memory serves me. there is another product as well you might see called Flatworm Control but I don't personally think it is as effective as the Exit. As a former President for MASC I got calls on this at least once a month. There is one other thing you might use that would be interceptor. It is a Prescription med that vets use. But I have a problem with it as the dosage seem to vary from vet to vet.

Disclaimer LOL

These are my opinions and views take them as you would any other source and verify prior to putting anything in your tank

Be Safe
Mark