can't swim up

My betta fish has been sick for about a week and a half now. He seems very bottom-heavy and lays on his side a lot. His gills are also VERY red and inflamed but it can't be ammonia or nitrate poisoning because I took a sample of his water into the pet store and they tested it for me and it came...

Housing What size is your tank? 5 gal What temperature is your tank? 78 Does your tank have a filter? Yes Does your tank have an air stone or other type of aeration? No Is your tank heated? Yes What tank mates does your betta fish live with? Plastic plants Food What type of food do you feed...