Exactly. So you don't kill them with the Levamisole. Instead you have to manually remove the worms/ larvae/ eggs so it's very difficult with substrate in the tank. Better off moving the fish to a bare bottom, treating them there a few times and removing the worms/ larvae or even better, move the fish again. I had to throw out the substrate to get rid of mine. If you don't, you will often relapse after a good long time. It took about eight months for me to get re-infested after the first treatment. Second time I treated the tank, then moved the fish (after the medication would be effective) to a bare bottom. I then treated again and then moved them back to the now freshly cleaned original tank. Problem solved.
I totally disagree with this. I'm sorry if you actually had to do this but if you get pure levamisole I put in just the front part of a teaspoon for 55 gallons and they were gone in two days never to return. I tried all kinds of these other treatments. Even lost fish due to crazy treatments that didn't do jack.
I did a good vacuum then treated a week later as directed and I've never had an issue again. With any worm for that matter.