Tail biting

UGH. Dx Dante won't stop chompin his tail! i did what's been suggested before, changing his decor around and giving him new things, keeping him entertained from the outside as well, trying to keep him as stress-free as i can, and still he chomps. D: i've not yet caught him chomping, but i know he is biting, cause it's not like fin rot, it's a few chunks missing from different spots. i keep his water clean, changing it every three or four days, and add aquarium salt to it every water change, to keep things clean so the chunks will heal without infection.

is there any other ways to stop a tail biter from munchin on their tail? D: i keep him stress-free, and keep things interesting inside, and outside his tank. i miss the pretty black on the outer edges of his fins(not like tail rot, more like the butterfly marking, only in black). D: