well 100% every two days is definitely ALOT. I doubt the fish minds though, unless you are handling him too much.
It should be perfectly fine for you to do a 100% once a week. I would highly recommend for you to grab a test kit and work pit the best water change schedule for yourself.
Did you make sure to keep the new water the same temperature as the old?
Fin rot is usually a gradual recession of his fins, where tail biting are sudden chunks dissappearing, and no black edges.
If he is tail biting after a water change, it could be because he doesn't like being handled so much and is stressed. otherwise it might be because the filter is too strong and he is biting his tail to cut off the extra weight slowing him down. In that case, try baffling the filter so it creates less of a current.
Sorry, I'm in bed and half asleep, ask me if I didnt say anything clearly, I'll try to help the best I can tomorrow:P