75*F should be good... I would also put the bubblestone back in to increase surface agitation, as increased O2 levels seem to help in treatment. Do large, daily water changes: clean water helps a lot.
Here is a link for a site that discusses treatment: http://guppyplace.tripod.com/Ailments.html
Here is a (bad) pic of one of my female bettas that died of columnaris:
The pic is fuzzy, but you should be able to see the patterned color loss on the fish. This fish went from looking healthy to this condition (and dead) in less than 2 days. Columnaris is a NASTY disease.
I noticed the following symptoms with my fish:
-heavy breathing ("panting")
-loss of color in patches (saddleback)
-deteriorating/shredded fins, beginning with the caudal fin
-rapid deterioration and death
I was told by a rather knowledgeable person that treating with Furan 2 could help, but II had to order it and wait for it to come in when I had the outbreak in one of my tanks. By the time it arrived, the fish seemed to be responding to the non medicine treatment I listed above (increased aeration and large water changes).
I have also heard to add salt to the tank, but I couldn't because I had corydoras.
I wish you luck, and hope it isn't columnaris. If you aren't seeing the "saddleback" like deterioration, it might be some other kind of fungus.