Sorry for note writing earlier...I haven't checked my messages for a while.
I just started keeping bettas about 10 days ago. A few things I do know...
1) Bettas will eat themselves to death, so don't feed them more than 3-4 pellets a day. Also, once a week it is a good idea not to feed them anything at all (just like other fish).
2) As far as possible, don't use a HOB filter...sponge filters are perfect.
3) Clean water is the best medicine in most cases.
4) What is your set up? Bowl / 5 gallon tank? 10 gallon tank?
5) Do you have a heater?
6) Are there any fungus spots?
I hope your betta makes it through...these are gorgeous fish.