How long do I need to feed my goldfish shelled peas?

I have two black Moors and two orange & white fantail goldfish. One of the orange and white ones has always floated on the top in his spare time for the last two + years that I've had him. I recently moved them all to a 50 gallon tank from a 9 gallon and the floating is more visible. I started feeding them shelled green peas and they love it and then I noticed the one floaty wasn't floating. Like the peas weighed him down. I did this for a few days then went back to flake food. Poof! he was floating again. so I gave them more peas and he is swimming around not floating at the top. Is there something I can give him food wise or medication that will help him? Will the medication affect the other fish? He seems fine when he is floating. I do notice that his poop has a lot of air bubbles in it. Any suggestions? Thanks.