Two goldfish, sick. anyone a expert? please help?

Hi, i have two gold fish. They are very old now. id say about 2 or 3 years old. they are in a 10 gallon tank and one of them is about 5-6 inches, and one is 3-4 inches. My little one got very sick a couple months ago and started spinning and being upside down and stuff. Now he is just able to lay on the bottom of the tank. it looks like he cant use his back fins. He still eats and gets help from the other fish. its been awhile and we thought he wasnt going to make it because he was acting so weird but hes made it a couple more months now which is amazing. could it be because they are so big and the tank is so small? im scared if i move them into a different tank they will get scared or i will hurt them in transit. could he have swim bladder disease? or is it the small tank? please help. i have a bubbler for the tank, a filter PH and amonia testers and all the readings are normal. should i move them into a diff tank? and if so, how? thank you, im very worried about my fishies