Well, it is true that clean water will help but not “ in no time”. It takes a while. Depending on how bad they got, they may never look like they did. Hate to say that but would rather prepare you in case they don’t. If they do, we can be pleasantly surprised. I do large water changes in my betta tank every 2 days because they live in a divided 10 gallon tank. I have come to the realization that nitrates over 10ppm can begin eating at their tails. Until you start seeing some regrowth, ( Clear skin), I would do a daily water change of about 75%. I also add a teaspoon of dissolved aquarium salt when my betta has tail damage. We can debate all day long on the benefits/risk of aquarium salt but I am a big believer in it for fin repair. You didn’t put any of that Moss in the tank, did you? Lol. P.S. - Can we get a pic?