Betta fish will not eat

Please help me! I have had trouble with my betta fish, named Nollie, since day one. When I first brought him home about a year ago he looked like he might die so I returned to the pet store and aske for advice. They said that he was cold so I put a heAter in the tank with a thermometer and he loves it! He was healthy for so long, but lately he has stopped eating and just floats around motionless. I have tried different types of foods yet he still stayed in the bottom corner of the tank. I noticed slight fraying of his fins, and a few red spots on him. I looked it up online and I believe that he may have fin rot. I went to the pet store a few days ago and bought an antifungal fish remedy, and some aquarium salt. I changed his gallon tank like I always do, and bought all new rocks and plants to make sure he was all clean. I added the suggested salt and remedy, and now he floats at the top of the tank instead of the bottom with his mouth poked up at the top of the tank. I am going to purchase a 5 gallon tank shortly, but should I wait until he is better? Will he ever recover? Please help me! My mom bought him for me a year ago to help ease my anxiety, but now all I do is worry about him! I sit next to his tank for hours waiting for him to get better but no suck luck. I even bought new pellets for him but he refuses to acknowledge the food so then I just scoop it out. Please help nollie :(