You may want to use garlic to boost their immune systems since it shouldn't do anything to mess with the other meds. The best way to do it is to buy fresh garlic cloves and mince it at home. Then soak it in a tablespoon of aquarium water for several hours. Next you wanna soak their food in it for maybe 10-15 minutes (depending on how soft or hard the food is).. If they're still eating they should gobble this up.
P:S You could buy pre-minced garlic but it can't have any acids in it that they use as a preservative. So it's usually safer to go fresh as I've seen some that don't say acid in the ingredients but still have a tiny amount in it which is enough to harm the already weak fish.