according to the flatworm exit directions, if you still have flatworms after their recommended dosage, you probably have a type of flatworm that is more resistant to the flatworm exit.  They recommended putting some of the flatworms in 1 gallon of tank water and add a drop and observe, and add a drop and observe until you see them dying.  that way you will know exactly how many drops per gallon are needed to kill them.  i had no adverse affects from the treatment.  corals didnt even close when i dosed it and featherdusters and other worms in the tank didnt seem to be affected either.