I thought my betta had fin rot as his tail became ragged soon after I bought him so I moved him to a hospital tank and his tail started getting better, so I put him back in my main tank but now I think it is getting worse again.

In the hospital tank I have no light and he never once flared, however when the tank light is on in my main tank he catches sight of his reflection a lot and flares, so it seems to me his damaged tail is being caused by him regularly flaring. I have tried to disguise his reflection but I still have to keep my room light on at night or he will flare at the front of the glass.

What can I do about this? I dont want to take him back to the shop as I love his and his personality, but I cant treat his tail as I dont have room to set up the hospital tank permanently.