I used to treat finrot and ich with non iodized salt, I tried a heap of meds and they seemed to work for awahile then when I stopped using the meds the ich would come back,
You will have to take all your plants and inverts out of the tanks and keep them somewhere else, You have to gradually acclimate the fish in your tank to the salt water and when the ich or finrot is gone you have to gradually acclimate them back to fresh water, acclimation would take me like 2 days in a 55 gallon tank,
This might not be method you want to use but Its just an idea, and it has worked for me in the past and if I run across it again I``ll do it to mine...
Maybe take the one sick Betta out and isolate her and try it with just the one before ya do your whole tank.
hope this helps