I used to use medicine that contain Malachite Green to cure fin rot. I dosed the full recommended amount at the 1st treatment, dose half everyday, change the water alternated day.
Didn't use a heater though...but i alternate the treatment with Malachite Green and salt (0.3%). I alternated the treatments every week...like 1 week malachite green 1 week salt. It took awhile to stop the rotting and then only start curing.

Good Luck