I'm sorry you're having problems, but welcome to AC. We'll try to help you & your betta. Please answer Bob's questions...& then mine too.
Like Bob said, lots of clean water can work miracles. Every day if you can, but do your best. It can be hard to tell if fin damage is from bacteria or fungus but bacteria is more common...Also healing fin damage can look white too as the new tissue grows.
If you can, post a pic or 2. It's hard to tell by pics but better than just a description of what might be wrong. Both of your fish & your tank decor etc please. Sometimes decor, tankmates or water quality can cause fin damage. We're nosy for a reason.
Yes, please post all your water test #s. If you don't have a good 1 like API then at least take a water sample to a shop. We need real numbers or at least a range...OK is not a number. Strips aren't great but I've used them. Write down the results & tell us.
We also need to know everything about your fish & tank! & then we'll ask more. How long has this "rot" been going on? Did you try any treatment yet? Does the fish eat & poo normally?