Fancy Goldfish floating on side

Have two fish in a bi-orb.

Today one of them either sinks to bottom motionless or floats listlessly
to top.

Occassionaly he swims about but all fins seem droopy.

He is still eating.

Complete water change 3 weeks ago

Other fish fine

Any ideas welcomed Thanks

I'm sure there will be many people who will reply to help you with your
fish but you need to give more details about your tank and your fish.
Please tell us how big the tank is, what type of filtration you have,
are you using an extra air devices? What is the temperature of the
tank? Do you use a heater? How many fish and how big are they? How
long have you had them? How do you perform water changes and how
often are they done? If you have test kits it would be helpful to kow
the ph, ammonia, nitrite, and nitrate levels.