Betta Fish: Sick Fish

I posted my betta’s transformation on here and mentioned he’s been sick. I’m not sure what to do and I’ve tried everything. He’s very lethargic and sinks to the bottom of the tank/lays on his side. He used to be in a community tank but I moved him out since he got sick. That tank was heated, had an air stone, water parameters were good however I did notice a .25 ammonia spike. I still have that tank and currently it has 0 ammonia. I do weekly water changes at 20%. I will attach pictures of the tank Picasso is in now. It is 10 gallons, just tested the other day (haven’t been able to again because of work) and it was 0 ammonia, PH of 6.8, Nitrite 0, Nitrate 0. He still has an appetite, perks up when I am near, can swim upwards. No bloating, no lumps or bumps, previously though, he did have a cloudy/slightly enlarged eye which went away with an epsom salt bath. Someone suggested a cycle crash, so I did a water change with seachem prime and aquarium salt to treat. Previously I had thought it was swim bladder so I fasted him and nothing. I treated with kanaplex and discontinued because there was no improvement after 3 days. After his first epsom bath he improved so a couple days later I did another, which seemed to be too much stress so I haven’t done one since. Any advice would be appreciated. Google has also said he could just be old.