I Have a Black Moor Goldfish Who Is Constantly Having Air In His Stomach.?

My Black Moor, Po (pronounced Poe), is constantly at the top and it seems like he can't swim down. The person at Petsmart told us he had air in his stomach and we should feed him peas to get it out. We are currently feeding him peas and that stops it for a while. Is there any way to completely fix this? Thanks in Advance!