ive never actually had to do it myself but from reading some threads in here theres two ways to do it, the first is to treat their food which will remove any worms or parasites inside the animals itself but doesnt treat for any larvae or eggs that may be present in the tank itself, the second is to actually treat the water in the tank which should take care of parasites inside the fish as well as anything in the tank. usually you treat with a 3 day course and then 2 weeks later another 3 day course to take care of any stragglers and larvae