Long time lurker here. I bought this betta about a month ago. I haven't had one since I was a kid so I'm not a genius when it comes to diagnosing diseases. He had a black outline to all of his fins and I asked the store clerk if he had fin rot. She said that they were just some of his markings. I came home from a day of shopping and now his tail looks like this: http://i.imgur.com/sH8Zn.jpg
(Sorry, I don't know how to post an image without making it huge).
It was fine until this happened, except for that black outline that I noticed. It never got any bigger. Just this. Is it fin rot? There's nothing sharp in his aquarium that could have done this. :/