Without really knowing what is actually wrong with this fish the only thing I can suggest is keep the water as clean as possible. Rinse the filter media in some tank water and keep the substraight free of debris. Add one tablespoon of aquarium salt to the tank this will help with respiration and help regulate osmotic pressure. Keep the salt level constant re add any salt removed buy water changes. If you do a 50% water change add a teaspoon of salt back into the tank. The key is to keep the nitrates as low as possible and stress the fish as little.
