Please Help Re: Betta Red Edges/Holes inside fins

Hello,

I need help with Betta fish.
It is my first Betta fish.

I didn't know anything about cycling when I got him.

I bought him about a month, a month and a half ago.

I got him a 2.5 tank and was told to change water once a week (about 25%).

He was fine there, making bubble nests, generally happy for about 2 weeks.

I upgraded him to a 10G and that is when issues started. He started to glass surf constantly without a stop.

About two weeks in he stopped, but by then I noticed he had lost some of his fins and they had black/ragged edges.

I was cycling the tank using Bacteria and followed all of the instructions and was measuring two different brans of test strips, as well. When his nitrites levels were elevated, I did water changes (daily). I also took the water to the store which sold me the fish and they told me my tank was cycled (that was about 3 weeks later), and that everything was fine.

But because he continued to have fin rot I contacted a vet and they prescribed for him vitamin C and Sulfatrim. It worked for about 2 weeks, he was recovering. Then, I cleaned his tank as usual, but I also cleaned his filter (in Aquarium water). After that, the next day he lost majority of his bottom fin.

I have been managing his fin rot since, he was fine about a week, but he started to get red edges (I gave him Prime in case it was ammonia), and inside his fins there are holes and they have a black outline now and are getting larger.

That hasn't happened before so I moved him to a hospital tank today, where he has Roobois Tea, Seachem StressGuard, (Prime - detoxed), a heater, and I gave him 0.25 Sulfatrim tablet.

Can anyone recommend me any help? What can I do now onwards to help the fish?

Thank you, everyone.