Betta fish fin rot?

Whelp you must know very little about the nitrogen cycle. You killed the tanks cycle so now you are killing your fish." Not necessarily.

If the OP kept the filter running in a separate bucket and didn't nuke it with used tank water... Unfortunately, not many people who ask questions on here know these tricks. OP, can you give us the exact ammonia, nitrite and nitrate readings? That's the easiest way to tell if your tank still holds the nitrogen cycle.

But surely, the 100% water change was a very bad idea: you could have solved the problem by doing multiple 30% water changes. You always loose some beneficial bacteria when you pester the filter. And a full water change is always very hard even on fish: he had to get used to brand new water parameters.

Acclimation is risky for healthy fish, let alone with a sick fish with a compromised immune system. I'd love to see a photo of your fish (sometimes, it tells us a lot), and I'd like to know if you've seen any other symptoms: loss of color, weird poop, weird scales, spots, swollen abdomen etc. "Considering he still has an appetite and is aware of me moving around the tank, he's not just laying around waiting to die..." Unfortunately what you describe, sounds really unhealthy: loss of buoyancy, lethargy and poking him with a stick doesn't count as aware of it's surroundings. Swimming and eating means that the fish is still fighting, but judging by your description yours is likely to need some help to heal.

If I were you'd get to the local fish store, get a broad spectrum anti-biotic medicine (avoid -fixes and other herbal tonics), Indian almond leafs or organic, unflavored rooibos tea (both are immune sytem boosters), and maybe some epsom salt too (check the pharmacy) before everything closes for Christmas so you have them at hand. At worst, you don't have to use them. "I'm thinking he's acting this way because his missing tails make it difficult to swim."

It really depends on the amount of the damage done. I wouldn't even try to judge without seeing a photo. (My betta is a tail biter, he usually has about half of his tail.) You might want to repost this question later for more answers: Y!

A has been broken lately, questions disappeared rapidly, it might or might not be fixed now.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.