Sorry about your betta! Having a sick pet can be emotionally taxing. FWIW I highly doubt letting the tank drop to 72 degrees had anything to do with their poor health. So I wouldn't let that thought run rampant.
As for treatment, I recently treated a betta of mine for fin rot by making medicated food and feeding it to him directly via a small dropper. I find this to be far more effective than just dumping the antibiotics into the tank. As for making the medicated food, I use a Repashy gel to make up a couple tablespoons worth of food or so (it's a very small amount). I let it cool slightly, mix in a few pinches of erythromycin, and then set it in my fridge. I feed this to the fish daily for the next two weeks. This is how I deliver all internal fish medications, and my success rates for treatment have skyrocketed ever since starting to do it this way.
Best of luck to you and your betta, I hope it works out!