with an ee, the more likely problem would be the filter flow to be honest. i've never heard of a betta biting off scales, i think you're right that he must be scraping them off on something. if he's doing better without the light, by all means leave it off. usually bettas are ok with light as long as they have a consistent day/night cycle, ie you turn the light off after 8-10 hours every day, but it seems possible your betta just doesn't like it. a picture now would actually still help, because we could perhaps tell if it looked like he was biting his own fins, or if they were being torn by something else.