Pictures and video of the fish?
What are the symptoms?
If a fish is floating up to the surface when it stops swimming, it probably has air in its intestine. This happens if fish eat a lot of dry food from the surface. If this is the case, stop feeding dry food for 1 week and use frozen or live foods instead. If it fixes the problem then the dry food should be reduced from the fish's diet.
If the fish sinks to the bottom when it stops swimming, then it has a swim bladder problem. There is no cure for this and the fish are normally euthanised.
If the fish has ballooned up overnight and has stopped eating, it has an internal bacterial infection and is dying. It should be euthanised.