I found this online and was wondering if this is even correct.
Do a full jar water change. Preferably provide a new jar. You must combine several medications to have a chance to stop this thing. Use Ampicillin and double the dose if needed, and use at the same time tetracycline (included in our Betta First Aid Kit). These medications usually come in capsules. A full capsule usually treats 10 gal of water. So for a 1/2 gallon of water, open the capsule and take the right proportion of powder and sprinkle on jar water. You may steer gently with a disposable plastic spoon. Tetracycline might turn the color of the water to a dark yellow or red. Overmedicating a little might help! Change water every third day and add a new dose of same medication. Continue until fins/tail stop receding and start showing some new growth. It may take up to 4 weeks to work, so donít give up. Once healing starts you should treat for one more week to be sure. Then you can ease up on the heavy medication and switch to BettaZing (included in our Betta First Aid Kit), which will prevent the bacteria from multiplying again. Change jar and sanitize old jar every week until healed.