Try it with 1.5 as RD stated only because your dealing with different stock than discus. I've treated up to 5g with no ill affects. Retreat in 2 weeks. Are you treating the whole tank or just the QT?
Dose as directed. If you are going to use this use this alone don't switch, follow the full course of treatment. After 24 hours you can added some Epsom salt to the water to purge dead worms. Follow water change schedule as recommended per product instructions. This should resolve the infestation.