How did you cycle your tank? Had you tested Ammonia and Nitrates also?
Sorry to hear of your loss. I know it's hard to lose something that you've invested so much time into.
Congrats on the find!! I would get a bigger filter once you start fully stocking the tank. Be sure to check how well it holds water and check the seals especially if it was sitting outside.
Water changes and treating with salt, may help. Also lowering the temperature may help. Take a look at the two articles.
http://freshaquarium.about.com/cs/disease/p/columnaris.htm
http://www.aquatic-hobbyist.com/profiles/disease/freshwater/columnaris.html
If you want try selling it in the classified ads section? What about like decoration in an outside garden? or just in pots with plants it will help with drainage. My grandma has a stone driveway and we've just been adding my stones that I don't want anymore to that. It adds a nice bit of...