I remember reading on another goldfish forum that; if one doesn't properly age their water for at least several hours(I think it was, could be longer) prior to adding to the tank, there can be (certain)gas bubbles from the pressure of the water being in pipes(it was to my understanding). These gas bubbles cannot be metabolized(or w/e) so they appear on the fish as bubbles.
For the sake of curiosity, do you add water directly from faucet to tank? if not, how long do you let the water sit? From where is the source of your water coming from?(does it travel a far distance to your faucet?)
Wish I knew the name of the gas bubbles the poster said, if I find it I'll post it.