I have had this betta for a few weeks now. I am wondering if he has fin rot. He currently has a black stripe going down the top of his tail and another stripe on his bottom fin. The tips of his fins look fine to me. He has been swiming and eating fine. He is in a 3 gallon tank with a filter and heater. The temperature has stayed 80* since i got him. When i saw the black stripes, I did a 80% water change. Here you can see some of the black.