Goldfish is floating! help?!?

It looks like my goldfish has a swim bladder problem. It's backside is floating, having a hard time swimming, stays near the top. It's in a 10 gallon tank with two other smaller goldfishes as well. They are all about 1inch long. I've tried peas but it doesn't want to eat it and the peas sink to the bottom. I want to try fasting but not sure if I'll have to make all the other fish fast as I don't have another tank. I've replaced the water, added dechlorinater and cycle solution. I'm out of ideas. What should I do?