Search for Hexamita (the disease), Metrodinizole (The medicines, spelling???) or metro on this forum. Its available at most "human" drug stores, depending on where you are it may require prescription. What I do is a wet a couple of pellets and roll them in powdered metrodinizole. I forced feed the fish, there is a recommended dosage for this, so there a little bit of guessing on how much to give your fish. I gave my 7 inch fish 2-3 pellets a day and in less than a week, he is cured.
Some dissolves the metro in the water, but this takes longer and its a hit and miss as metrodinozole works best when ingested