This is interesting. I've read on Petco.com that halfmoon bettas have the most fragile tails of all the betta tail forms, possibly because of its huge spread. This would account for so many HMs with ragged looking fins and tails. Maybe not every supposed HM is biting his tail. Maybe the tails are so brittle they shred on their own. I know that gradually my HM's fins have gotten more and more ragged, including his dorsal which I imagine he can't bite very easily. But no signs of fin rot at all. They just seem to tear on the slightest thing. What does everyone think?
This is from their webpage for the HM.
"Male Betta Halfmoon
Also known as Siamese Fighting Fish, Combattant (Canada), Trey kroem phloek (Cambodia), or Cá lia thia (Vietnam).
The Male Betta is well known for its jewel-bright colors and spectacular fins. Bettas are anabantids (Derived from the Greek Verb anabaino, meaning "to go up") which physiologically means they have two methods of breathing. Bettas breathe from their labyrinth organ which enables the fish to breathe from the surface. Bettas also breathe through their gills like other fish.
As the name suggests, the Male Halfmoon Betta has a large tail in the shape of a halfmoon. The angle is from 170° to a perfect 180° tail span. This tail variation is one of the most beautiful, yet fragile of the Bettas that has been bred within the past decade or so.
Bettas will "flare" their fins when disturbed or threatened. The male Betta will attack another Betta and have been known to attack similar-looking fish. It is recommended that only one male Betta is housed in an aquarium with plants and very little current."