Bettas need help.

[CrowntailBetta]

I went away for the weekend and when i came back the heater in my tank wasn't working and the water had dropped to 66degrees. At first I thought both my bettas were dead, they were both totally still just under the surface. I brought the water temperature up over the day and its now back in the low 70s. Both fish have really bad fin rot, I took the crowntail that seemed in a better state out into a 3/4 gallon jar and treated with melafix, the other veil tail betta which was clearly nearly dead and who has had about a third of his finnage turn black now has the whole of the four gallon tank which i have also treated with melafix. Neither has eaten much and my baby bristlenose which was in the 4 gallon with the veiltail has died. Is there anything else I can do.