Betta Fish: Sick Fish?

I was gifted my betta, King, a month ago along with a tank and a few decorations and other necessities all new from petsmart. I live an hour away from petsmart and also kinda wanted to keep a pet so I accepted and did a bit of research and set up his tank. I used a bag of that activeflora? or whatever it's called, and waited around 6 hours before acclimating the fish since it said insta-cycling (possibly my first mistake trusting any claims from a chain store). Now that he was out of the blue water in a cup I noticed he had a brown patch all along the edge of the tail fin(?) He has yellow on him so I though it could be discolored from the methelyne blue. A week after that a patch of algae started growing on a side facing the window a cleaned most of it but left a small patch as I was researching snails and shrimp. Fast forward to beginning of July and I just notice the algae growing on the filter casing and King's fins shredded all where the brown patch was and the patch had spreaded a bit around deeper slits in the fins, now I get concerned and take out any of the decorations that I thought could cut him,(which was most of them. Why you can't see any in the tank), cleaned all the algae, did a 60-70% water change(number based off memory) and began looking for solutions. I ended up getting some almond leaves and melacure (mistaked it with Methelyne blue somehow) and started using them Which seems to have helped significantly and reduced the brown patches on his fin. I thought it was only fin rot but this morning his head had white on it but after he swam around a bit it faded away so I think it was his slime coat but not entirely sure. Despite all this there has been little change in his behavior the only noticeable ones I've seen is his top fin being on one side more often and him rarely turning sharply so that his face is close to his fins which makes me believe he's trying to bite his fin. I mostly made this post because I was scared of the white patch but once it went away I decided to ask online anyway to see if anyone could spot anything else wrong.

He's in a 3.5 gallon tank right now but I'm looking into upgrading sometime soon
Nitrate was zero when checked with those api strip ( my kit never arrived) but nitrite was around .5 I'm pretty sure it was near the opposite last time I checked, is that bad?
Temperature was 79-80 last time I checked
I used to do 30-50 percent biweekly water changes but started doing 20-30% every day after using three drops of the melacure and letting that run for an hour.
His diet is ~4 Pellets and half a freeze dried bloodworm
If you'd like a better picture feel free to ask.