I bought a betta, and unfortunately neglected to quarantine.. resulting in the death of it, and it's tank mate (I have a divider!) of fin rot.
I kept the tank free of fish for about a week.. and then added a fresh pair. Now one of them is showing signs of fin rot.
I would change my water, completely, but unfortunately, I'm in the middle of the city, and have horrible amounts of chlorine.
I suppose I'll pour some from the tap tonight and let it sit.
I didn't think that the disease would stay in the tank without any fish present..