Betta Fish: Anything else I can do for Bluey?

Bluey had gotten worse and worse and someone told me it was fin rot. I had him in a hospital tank and treated him with Kanaplex in 3 doses. I didn't do his main tank because there were 3 small shrimp in there. The smaller tank (1.5 gallon) was harder for me to manage because it kept spiking in ammonia and even after doing water changes, which I felt were traumatizing him to remove or to leave him in the hospital tank during, it wouldn't change too much. On his last day in there, I was so scared as after having done a 75% water change the day prior, in the afternoon when I came home he was extremely lethargic, barely breathing and sinking to the bottom. I thought he had died. I immediately put him back in his 15 gallon tank and he started swimming vigorously and breathing again. His fins were clearer on the edges as well, so I thought his fin rot was all healed.

While he definitely improved returning to his OG tank, I am now seeing new small tears still which make me think there is another culprit – potentially his filter has this sponge that could be roughing him up. Someone also told me it could be him nipping his own fins, which I've never seen but supposedly you don't see it happen. He rests at the bottom a lot more now then before the hospital tank incident, but I get happy whenever I see him near the tank even if he's not vigorously swimming about like he used to. He energetically eats 6-8 pellets a day and will literally snap the water for them.

Additionally, my tank started becoming infested with hair algae. I didn't know it was bad and thought it was moss at first. I started removing it daily but it is growing on my moss which is much harder to remove.

1) What do you recommend to use for the sponge on the filter?
2) Anything else I can do to improve his QOL? His tank conditions are perfect, and I do weekly water top ups/biweekly water changes.
3) Best way to remove hair algae?