Fin rot or Tail biting?

[folion]

OK so at first I was thinking this was fin rot so I started daily water changes and tested water every day before the changes.
The post I made about the fin rot is here.
http://www.bettafish.com/showthread.php?p=533383#post533383


Ok so after the incident talked about in the last post. My betta's tail was rapidly healing. But one day I noticed major chunks of his tail missing. It seemed all happen with in 2 days and his tail was rapidly falling apart. After posting about the problem I did a large water change, all ammonia nitrites and nitrate at 0.0ppm sense then with daily 10% water changes (salt added to the new water.) I also added API stress coat. I also added more protein to his diet.


After doing that I once again started to see his tail heal and regrow very fast. Then today I got home from work to see another massive chunk missing from his tail.


He is eating really good. Swimming around happily when I get home. He does not seem sick at all. I just don't understand what is going on.


Any ideas?