Betta Fish: Whats more can I do for him?

This is my boy Pirin, and like 2~ weeks ago my filter broke and I couldn't use it. After a week I was finally able to get a new one (sponge filter <e jet 102>) I set it up 6 days ago, and I was expecting to see his fins get better but it doesn't look any better, maybe I'm out of my mind but it looks worse? the filter is set to be with a very low flow, but if it's needed i will record it and send it here for second opinions. I did a water test yesterday ( and one 3 days ago and another one when I set the filter up) they both showed the exact same things:

Ammonia 0

Nitrite 0

Nitrate 0,5 if i remember correctly

pH 8.0 (ik it should be lower but I'm too scared to change it incase I shock him)

The tank size is 10g (38l)

Temp is 25c (77f) , but I'm probably gonna up it to 26c (78f) if that's recommended

Diet consists of:

frozen artemia (2x a week)

frozen blood worms (1x a week)

frozen daphnia (1x a week)

bug bites (2x a week)

and one meal skipped (consistently on Sunday)

(The daphnia is aways given after the day the bloodworms are if this is even necessary to know. His diet isn't set in stone other than this and the Sunday skip, as I know they like diversity and im hoping the changes give him entertainment or smt ( ;´･ω･`) )

other than that im not sure what else to add, I've had this aquarium for 1 year and the fish I've had since 12th of December. His fins were better before, he got sick with fin rot not too long ago and I used eSHa 2000 to fix it, which actually work very well.

I dont know what more I can do, i will buy anything, as long as it isn't found only in America or Western Europe because the arrival time will be insane (as well as the price) i have a store that sells mostly western fish stuff tho so I'm gonna have a look. Brand of some products might be different.

(Also the tank is heavily planted if this is of importance as well)