If you are treating the fish for tapeworm or gill flukes, you do a big water change and gravel clean the substrate 24-48 hours after treating them. Retreat the tank once a week for 3-4 weeks to kill any baby worms that hatch in the fish.
If you do water changes on Friday, then treat the tank on Wednesday night or Thursday morning, then do a big (75-90%) water change and gravel clean on Friday. The gravel clean is to suck up any worm eggs or bits of tapeworm that have settled on the bottom.
Remove carbon from filter before treating otherwise it will remove the medication from the water.
Praziquantel only treats tapeworm and gill flukes. It does not treat round/ thread worms.