It is unlikly that these bubbles will harm the fish.It is however not desirable as far as looks go. It can also cause the evaporation to increase.Try wrapping the return in a foam filter.This should stop most,if not all these bubbles.
Some say that those bubbles are one of the causes of popeye, I don't know how true that is. You can try running your return into a sponge or some carbon, this will trap the bubbles. Good Luck