I am not betta keeper, but lots have been written about them lately. First, some questions: What is the temperature of the tank(s)? How long have the tanks been up and running?
This is from Christmasfish from another thread, "Betta white stuff" in the unhealthy fish section:
The favorite method here for recovery and treatment is heat..
For ich and faster recovery of other things. It is an oldy but goody.
And it is the most effective in the long run. Meds are hard because you have to know for sure what you have or you will just end up giving the poor fish heavy metal poisoning without ever touching the pathogen.
And the disease is strong it will allow a med bath to take effect.
And heat is best in your situation; because of the medicated soup that "backyard" betta breeders keep their crowded fish in and the the poor treatment on their way to retail locations..
There are also two new strains of medication resistant and betta-first ich and gram negative bacteria running like gestapo over some well known and healthy lines. Columnaris is getting to be seen more often than before in bettas (resistant #3?)
Piper, I hate to be the bearer of bad news, but your bettas may simply be too weak to fight this. I can truly sympathize with you, I am having a Badis crisis--I owned 6 of these little fish for over a month and they are now dieing
. I have done everything right, but I lost three since Friday and another is not looking well. I wish I could be of more help.