Fin Rot

My betta fish, Malcolm, has fin rot. I know I don't talk about him much and I have yet to take a picture of him to post in "My Family" section, but I still care about him. I feel terribly guilty that he has fin rot. Technically, it's tail rot because that's where the white cottony stuff is. How does a fish get fin/tail rot? Usually, by not having good water conditions to swim in. I know I'm a bad mom. I feel awful. Right after school, I ran around to two different pet stores looking for maracyn and sea salt. I even got him filtered water. I've already cleaned his tank and plants. I also boiled his gravel for a few minutes. Hopefully, everything will be okay. Don't forget to keep your tanks clean! >_<