Betta fish tail problem?

Hi. i have had a betta fish since october. Afew months ago, my betta fish got a large amount of ammoia in its tank because we didnt clean his bowl regularly. We quickly went to a local pet shop and found out how to fix it and now we try to keep him healthy. He has checkups using a tester for acid and arkeline and also weekly 3rd bowl changes. although, after the effects of the ammonia build-up, my bettas tail has become tattered and stringy. (Not shorter like the tail infection) and he also spends a while at the bottom of the bowl only swimming around when i come in. (because of food!) he also does not flare with his gills as he used to around the other fish. We also try to keep the temperature around 21 degrees (with a light) and feed him betta flakes (which are healthy for bettas) and his colour is mostly fine. I would also like to point out he seems to hide under ornomnets in the shade from the lamp. (problem?) anyway, thanks. hope for replies soon!