1. You should take out any carbon filters in the aquarium when you are medicating your fish. The carbon in the filter will remove the medicine from the water, so it is best to remove it all together.
2. Try to provide an environment that is as stable as possible, fluctuations in the aquarium can have a negative impact on the aquarium. Try to keep the temperature and PH levels fairly balanced in your aquarium.
3. You should feed your fish a variety of food and not just flake foods. Mix it up with fresher foods from their regular diet. Giving them more a varied diet will increase the general health of your fish.