BG wrote:
Three months ago our betta had a small bump at the base of its tail
fin. Now the bump has grown to about 4x4mm, sticking about 2mm out.
It's white and doesn't appear to have any fuzz on it.
http://static.flickr.com/120/313500599_677ffd3331.jpg

The betta seems to have trouble floating, as it works hard to get to
the top and spends most of its time on the bottom. It's about 2 years
old, and it appears and behaves normally otherwise. I have Maroxy and
Maracyn Plus on hand, but I haven't tried either, since I'm not sure
what it is.

Any advice?

Thanks!

I would treat with Kanamycin. Make sure your water parameters are good
(ammonia 0; kH 80 ppm or higher). Poor water conditions will exasperate
any treatment.
Other potential treatments include Nitrofuarazone or maybe Pimafix.

http://www.americanaquariumproducts.com/Columnaris.html

Carl