Betta Fish: This is fin rot right? What can I do?

I was trying to take photos of my betta when I saw he had a massive section of his tail gone in this photo. I feel absolutely terrible, and I don't know what to do. He's in an okay tank, I know its great, but its what I have at the moment until I can get something better, but I dont know when. I really, really don't want him to die. I'm going to try to save up for a better tank since I'm young and don't have a job, but my parents can help me get whatever treatment I need for the fin rot. How do I treat it? Is it easy to treat? Is the treatment guaranteed to help? What's the best option? I'm freaking out because I really, really love this fish, and already had a scare with him almost dying once. Any advice is greatly appreciated, sorry for the ramble