You might want to check the water [ph, ammo]. Have you been doing water changes? Filters alone are not enough. How large are the tanks. What are you feeding them?

Is this a fancy gold or a everyday feeder fish type of goldfish? If fancy type (ryunkin or oranda) they are sometimes prone to swim bladder disease.

You may also want to try to feed them cooked, peeled green peas (feed first thing in the AM when they are hungriest- sometimes it takes a while for fish to eat greens if they've never been introduced to them before) The peas help out with any stoppage if they are constipated.

This happened to my fancy guppy once, so I isolated her in a small bowl for a few days and she got much better and now she is the mother of 15!