Lump/floating fin?

Ok. My betta Jeff Klee is currently being treated for Columnaris. I caught it very early, sterilized everything, took great care in 5 days of treating, and today did his water change and am doing 5 more days, as I've heard to do 10 to be sure it is gone. His columnaris symptoms appear to be gone, but he is still a little pale.
However, he has a new problem, and to be honest, I was so busy worrying about the columnaris that I don't even know when it started. On the long, dangling ventral fins near the belly (I do not know the name so am adding a drawing), he has this... bubble/lump near the tip. It is causing the fin to go straight UP, which obviously is a nuiscance to him, and I bet is adding to his stress during recovery.

So... what on earth would that be? It reminds me of an air bubble, but in the skin; the fin doesn't look necrotic, damaged, pale, ragged, darkened, or anything. I haven't caught him or palpated it because I do not want to stress him, but it looks solid. Is there a parasite that can get under the skin and cause a bubble like that? Is it maybe a pocket of infection? Or a tumor?

I'm wondering if I shouldn't catch him, wrap him in a wet rag or paper towl, trim it off with surgical scissors, and hope for the best. I've heard of people doing this with chronic fin-rot cases, and if this is a tumor on the end of the fin, I'd be lucky to be able to treat it before it spread. But if I can just treat him for something else, I'd prefer that to traumatising him during recovery.

I couldn't get a pic of it, because the glass on his tank is "scratchy" and I just get glare when I try for a photo. So here is a drawing, with the big red fin going upwards being the odd thing I'm describing.

::blinks:: Um. Ok.
Jeff Klee is my Fin Biter/Self Mutilator and... I guess he decided to take a proactive stance. The offending fin is now chewed in half, and floating around in his cage. He was chasing it like it was a big worm while the current whipped it around.
Anyways, I took it out and its... kinda squishy? I poked it open but it wasn't anything impressive, some fluid, some tissue. No idea why the hell it was floating straight up like that.

If anyone has any advice though, I'd like to know in case he grows another one. And, because I neglected to do it on my first post, here is the other info regarding him:
Jeff is an uncycled 5 gallon filtered Eclipse Corner tank. It gets either full weekly water changes or bi-weekly 50% partials. The readings for everything are all within safe limits, the water hardness being moderately elevated but not in the dangerous range. The water is dechlorinated with Stress Guard, Aquarium Salt, and Blackwater extract also added. It has no real plants. The fish is currently on an anti-bacterial food and is on Maracyn powdered. His usual diet is frozen bloods worms and brine shrimp, three different brands of betta pellets, and one brand of betta flakes, with fasting 1-2 times a week, and an occasional frozen pea.

I would treat it as bacterial for the moment if it has fluid in it.
http/www.fishpalace.org/danscomments.html#Lump