I have an aquarium with two white goldfish With red heads, and I got them at the same time and they were both the same size, babies. Now one is about the size of my fist and the other about half that size.
I noticed the big one floating at the top a lot and hanging upside down. I looked it up and it's called swim bladder disease. He literally has too much gas! The cure is to skin and crush frozen green peas and him that. He loves the green peas, and after a couple of days, he seems to be improving. Hope it gets better. Who would ever think a fish needs fiber.