Can you check the medicatin bottle and tell me if any of the ingredient is
praziquantel, malachite green, methlyene blue, or acriflavine ? Those usually really helpful with external parasites. But the medication you are using right now i think i used long time ago when i had my first fish . And it helped , but i used it longer than it recommended though.
Are you able to raise the temperature up to 84*? It will help to speed up the parasites life cycle and they will fall faster. And some even said it can kill them. I know recommended temperature is even 86* But i am not sure if it ok to do it with medication. I would think 84* with medications and salt would be fine. If you able to do it do it really gradually so you don't shock him with drastic difference. About 1-2* every 2 hours.