I suggest doing a 25% to 30% water change everyday. Get a test kit by aquarium pharmacuticles from your local fish store test befor each water change. You can also buy some Melafix and aquarium salt that should help a lot. Also only feed once a day, and NO FEEDER FISH!! They carry hole in the head and have no nutritional value. For treats I recomend nightcrawlers/ worms, Crawdads.
The test kit that you buy needs to have tests for PH, ammonia, Nitrate, and Nitrite.
Never replace all of your filter media,simply rinse off in the water that your doing the water changes with.
Without knowing your water parimeters I cannot tell you for sure what the exact problem is, but this should be a good start for now. Hope this helps you.