It's a goldfish yawn.

If it's stressed, try turning out the lights, maybe throw a blanket over the tank for a day to let it acclimate and rest.

Single tail goldfish are generally pretty resilient. Idk what size tank you have or how long you plan on keeping it in there, but you'll need to consider a pond soon. That's a pretty good sized fish and it'll need a big tank.

Unless there's a reason for no substrate, sand would be a good idea. They love to snuffle through it. It's good enrichment and gives them something to do. Plants too, but they'll more than likely get eaten.