You can put chemicals in the tank, but you don't know how your fish will react to something foreign in their water. I really wouldn't do it. You need to clean up the water, so start removing and replacing half the water a couple of times a week and add a couple of teaspoons of standard aquarium to every five gallons of the new, treated tap water. Do a thorough job of vacuuming the bottom material when you change out the water. Reduce what you feed to a little every couple of days. You can follow these steps until you see improvement in the fish. Once the fish recover, I would change at least half the water weekly for as long as you keep the fish.
"Fear not, my young apprentice. Just change the tank water."