A photo would really help here, but I'll do the best I can to advise you. Sounds like fin rot or it could be scorching from the water chemistry I know you said the water is fine so it looks like fin rot it is. The white spots are a secondary parasitic or bacterial infection. I would relocate the betta to a strip tank, water, heater, air and fish-No carbon. I'd first take a gallon of tank water and add a tbs or aquarium or preferably safe rock salt and dissolve it. Place the betta in that mix for 20-30 min. If he looks distressed after 10 min. then move him as this indicates he is sicker than you think. If he is fine an extra 10-20min. is fine. Then move the betta to the strip tank / container and add a 1/2 dose Methylene blue and a 1/2 dose of Merichite green. The next day change half the water and repeat the second step of the process-medications-for 5 days.