This is long, so just skip to the last paragraph if you don't feel like reading, I'm just providing some background info. for those who like to play diagnostician here
I adopted my sister's betta because she kept him in a fishbowl for >two weeks without water changes or heat. Anyhow, the fish was in pretty poor shape - he was lethargic, had complete pigmentation loss at the tips of his pelvics, and minor fin damage - but those were present for weeks without any visible signs of necrosis. I've gotten him a 3gal KritterKeeper with a heater so old that it has a wad of asbestos material near the resistor coils - needless to say, I barely got it to fit inside the KK (by turning it into a fire hazard using sewing thread connected between two suction cups) and it wasn't very reliable. I ordered a new heater on amazon, but I accidentally sent it to my home address instead of my apartment (living near campus), so I didn't have a choice as the weather has been extremely cold for the past few days.
Immediately after putting the heater to regular use, the betta developed ich cysts - he had one on his mandible when he was in my sister's care, and I guess the reason they haven't proliferated before was because of the lack of heat. I treated him with heat, 100% daily water changes and ~3.5tsp/gal aquarium salt, and he was doing great; his cysts had been greatly reduced, and he was just starting to develop iridescent streaks across his caudal and anal fins too - I didn't know red-VT mutt bettas could be so pretty.
After one of his water changes sometime this week I decided to try a new position for the old heater to minimize the risk of appliance fire, and I guess the thermostat slipped or something because the water in his tank heated up to 86 degrees for a few hours. Then, maybe two days ago I noticed that he had white-ish patches along his operculum and white scales in the fleshy ventral section of his head directly anterior to his pelvic fins. I was worried for a bit, but the patches looked symmetrical and most bettas have heterogenous pigmentation on their heads anyway, so I didn't think of much of it.
Last night, I noticed that the betta's caudal, which had just begun to regain its pigmentation, had become transparent along the fringes and was lined with dead, necrotic tissue at its distal end. I remembered OldFishLady's post in a different thread warning about 'flex,' and I looked up the symptoms, and indeed it appears that my fish had contracted columnaris, probably because of unreliable heating. I responded accordingly, gradually lowering temperature, performing water changes, etc. I also moved him into a smaller QT, which happens to be a ~1gal tall bucket since it was the only thing I could find that would comfortably fit my carcinogenic heater, for the sake of maintaining a stable 76 degrees in the water.
Anyways, the disease progressed pretty fast, and I have no idea how it managed to get past 0.035% salt content of my aquarium water. He seems to be flicking his maxilla/premaxilla a lot (think yawning), and his caudal is getting darker and is beginning to form two distinct two lobes down the middle. His iridescent scales are now rusty brown in color, but he seems stable in QT right now, and overall he hasn't lost any significant portions of his fins. The white discolorations have increased in size, but there's no visible signs of white fuzz or lesions, and he's still surprisingly feisty; his appetite is still quite voracious and he still does his hunting routine whenever my fingers are near his tank, he even jumped at one of my fingers during his last water change.
I'm pretty far from any pet store at the moment, but I can probably pick up OTC antibiotics from the student pharmacy - I just don't know if there are any antibiotics that are effective for both fish and humans. I might drive back home tonight to pick up my new heater and an airstone, and maybe leave him there if he seems too weak to make the trip back - it's a 2-3 hr. drive both ways. I'm spending way too much time on taking care of this fish than I should, but as a biology major I would be really ashamed if this guy dies under my care. Anyway, I'm about to leave for class, I'll check this thread again on my way back in case someone has a suggestion for an antibiotic to use. Apologies for the long post and thanks in advance for any ideas!