There is a difference between fin loss and fin rot but it isn't always
very obvious how to tell them apart. Fin rot is a bacterial infection
and often occurs due to poor water conditions. Bettas are extremely
susceptible to it and it's not uncommon for even seasoned veterans to
have to deal with it from time to time. General fin loss or tearing
(pin holes too) may occur if your betta snags his fins on something or
from flaring. Fin rot can often be identified by black or bloodied fin
tips. Often chunks of fins will just slough off. They are often
characteristically tattered at the ends as well. The signs of fin rot
may be less obvious with darker colored or red bettas.

http://www.nippyfish.net/finlossfinrot.html