What is the temperature of your tank? Most medications don't work well or at all in cool water and the optimum temperature for most medications is between 82-84 degrees. If it is for ich the temperature should be kept at 85 degrees for about 10 days to make the parasite go freeswimming from the fish and die in the water so it can be kept off of other fish. Fungus infections can also be caused or made worse by poor water conditions. You should perform a water change and vacuum the gravel to remove as much debris as possible and check the nitrate, nitrite, and ammonia levels of your water to see if the water could be part of your problem. You also didn't mention what the size of your tank is or the type of fish involved are. If you could test your water and post the readings it would help us to help you further.
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When fish are sick, constant water changes are good. Many commercial medications will often not help. Aquarium salt is thought of as very therapeutic. However I am not experienced with sick fish, so wait for someone who knows how to help you. I hope your fish will be OK.
As for the water changes - when you're using medications, you have to follow the label as to the duration of treatment. After the treatment is finished, you should make a partial water change - as much as 50% would be good.
I think it's safe to combine aquarium salt with medications too. From what I have read on FishLore, 1 tablespoon of salt per 5 gallons of water is the right dosage. And add appropriate amount of salt with every water change (if the fish are still sick).
What medications are you using? There are many types of treatment you can try. I think the salt will help, along with the water changes. Maracyn I and Maracyn II together might help if you haven't tried that already. Salt can also be added with these medications. Also, make sure you use aquarium salt or epsom salt. I've heard too many conflicting opinions on table salt, and would avoid using it.