I still use the powdered, pure Praziquantel. I do one treatment, leave it in the tank for several hours, and then do a large water change. In the discus hobby at least, you're only using it for flatworms (namely tapeworm). If the fish have tapeworms, they should expel them within a few hours and you'll see them on the bottom of the tank. If they do have tapeworms, you can wait a week or so and treat them again.
Usually if the fish are razor thin I start with metronidazole. After a week-long metro treatment, I give them a break for a while, then do the levamisole.