Sounds like a swim bladder problem. Feed him peas for a few days and find a good sinking semi-moist food for him.(I would suggest bio-blend) Out of curiosity, did he go through a growth spurt right before this? Orandas are a breed that will sometimes end up with a permenant problem when fat deposites push into the air bladder. The short of that being that he may spend the rest of his life with no bouyancy. If that is the case make sure you move him daily so he doesn't get sores.
This could also be water quality, though. How often are you doing water changes? Do you know any of your chemical balances?You could take a water sample to your lfs to find out. How long has this tank been set up? How large are your golds? An average fancy gold will need a minimum of 15 gallons, so even if only half of your 40 fish are golds you're looking at a minimum of 300 gallons just for them once full grown. And even if they're small now they need that room to grow to full size and stay healthy.
As a side note you shouldn't mix other species with golds. Goldfish tend to be "dirty" and can hurt the other's health. They also prefer a lower temp than tropicals. Then there is the problem that when the golds are big enough they will make a quick end to your swordtails, seeing them as nothing more than a yummy treat.

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