I found this whilst looking for something else; it probably isn't all that useful but I found it quite interesting....
(I've included the copyright at the bottom just in case).
Tail-Biting -- Why do bettas do it, and is it genetic?
By: Victoria Parnell
Submitted: 3/30/2006
One of the most disappointing things that can happen when you have fish shipped to you is opening that long-awaited box just to find out that your gorgeous, long-finned male betta is missing half of his tail.
So what the heck happened?? And is there anything that could have been done to prevent it?
The first thing you have to realize is that, usually, your betta did this to himself and most sellers can't take responsibility for what your fish does to his own fins while in transit. I'm not sure anyone really knows why bettas bite their fins, but based on common sense and experience we can wager a fairly accurate guess.
I've found in most cases that a male that chews his own tail is reacting to a stressful situation. This is why you most commonly see it in bettas that have been shipped through the mail, or those that have been left uncarded for too long. During shipment a betta is in complete blackness, in a very small space, and doesn't know what any of it is all about. On top of that, he's no doubt being handled very roughly. Because of the limited space in a shipping bag, the fins are being swirled around the betta's face, and he is in especially close proximity to the caudal fin when he turns around in the bag. It is almost understandable in that situation that he might reach out and grab hold of an object that brushes past his nose, not realizing it is his own tail. Tail biting during shipping happens primarily to very voluminous longfinned males (in other words, the ones that we most want to NOT damage their tails!), but I've seen it happen to females and plakats on occasion. In every instance, these had fins longer than was normal for their sex/type.
Bettas that are kept carded are also notorious for chewing off their own tails if they are suddenly placed in extended contact with another male betta. Sometimes it is an extreme aggressive reaction when the fish is unable to reach the other male and bites his own tail in frustration. In many cases the betta will actually eat the bitten portion, but sometimes you will only find pieces of the tail on the bottom of his jar.
Bettas kept in very strong artificial or natural light for prolonged periods of time may also react by biting their tails, and I believe they do it out of a similar reaction to being shipped: stress, blindness, and confusion. A betta's eyes are designed to see certain spectrums of color and movement in a very subtly-lit environment. Bright light makes them feel exposed and increases stress.
So, what can be done?
A bitten tail is an open wound, and vulnerable to infection. While your fish is healing, you should make sure he is kept in very clean water. I don't usually recommend medicating as a preventative, but a tiny bit of Melafix in the first few days will usually do wonders for encouraging new growth and preventing fungus or harmful bacteria from infecting the wound. Keeping the fish in a strong Indian Almond leaf infusion will have a similar effect, and the added bonus of keeping the fish pacified to prevent further damage.
There is really nothing you can do to stop your betta from biting his tail if he is so inclined, but there are a few things you can do to lessen the probability of occurance. Large-finned males seem less likely to damage their tails if they are shipped in larger bags with greater water capacity. Putting a small piece of Indian Almond leaf in their bags will seep beneficial tannins into the water that seem to calm them and make them less likely to succumb to stress. If you keep your bettas carded, try not to expose them to other males for longer than a few hours per day. Keep you fishroom dimly-lit, only exposing your betta to bright light if he is being examined or photographed.
There has been some speculation that the tendency for a betta to bite its own tail is genetic, and there might be something to that. You certainly do not often see a veiltail betta with self-mutilation, even though they go through much more stress, on average, than your showy HMs. Along with their fuller, longer fins modern show types may have inherited a predisposition toward nervousness and extreme reactions to stress. It would be interesting to see if the trait could be bred out by excluding known tail-biters from your breeding program. Its a fact that most tail biters are being used as breeding fish, since breeder-hobbyists can usually purchase them far more reasonably than unbitten males from the same spawn, and are not as reluctant to have them damaged while spawning. I myself have been guilty of throwing a big, showy male right into spawn as soon as he shreds his own tail, and I'm not alone. Breeders may be unknowingly perpetuating this trait, increasing the percentage of beautiful tail biters in the betta community. Something to think about!
A bitten tail should not be confused with a 'blown' tail. When a male is described as 'blowing' his fins, it is a term associated with the curious abberation that occurs in some heavily-finned males that are swimming or flaring too hard. The webbing between the rays develops pin holes and the ends of the tail begin to fray, like an old flag that has been whipping around in the wind. In extreme cases the caudal fin completely disintegrates, leaving the betta with spikey ray extensions and no webbing to speak of. In its milder form, tail blowing can cause the fins...all three unpaired fins, not necessarily just the caudal...to split along the ray in several places. Treatment for these fish is the same as for tail biting; clean water and watch for infection. Bettas with fin damage are more vulnerable to bacterial and fungal fin rot.
Bacterial and fungal fin rot also damage the fins, but it looks far different. You can observe fin rot gradually eroding away the fins in a fairly smooth and uniform pattern until the fish is left with nothing but stumps. There is usually a black or red edge in fin rot cases, and these should be treated immediately if you wish to stop the progress of the disease. If you have a fish with damaged fins watch carefully for the beginning of a red edge to the wound. If you catch it early enough, a few days' worth of treatments with Melafix will clear the problem up before it becomes a major hassle for you and your betta.
Although it's not pretty and can be disappointing, with proper care a bitten tail will heal within weeks and often return to its former glory.
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Then he got what I think was columnaris and died 
