Hi,
yes it is very common mine do it all the time u will notice a change in them when they get bigger also. If u do some research some of them turn a really beautiful reddish color & many other colors it's neat watching the changes. I have had goldfish for over 10yrs now and they are great.
Don't use Goldfish pellets they've got oil in them and if your goldfish don't each all of them they will kill your fish. The best bet is to get the flakes they won't harm anything.

Take Care,
Sheila