Well ANOTHER one passed away, I am losing them so fast now

I took all of them out of the tank and they are back in their cups. I gave them salt baths and now they are in their cups with some semi-salty water. I have them under a lamp and they are staying there overnight. Everythign from their tank is soaking in salty water too, even the tank itself >_< Tomorrow I will set it up again and if this doesn't work I don't know what to do.
Like the last ones there are no signs of sickness nor damage done by another Betta

I am down to four now! The one that is in its own tank is doing fine, as is my male. I have a feeling the original one had some kind of infection that is spreading to the others