there is probably air getting into your filtration system somewhere. Try to locate where and seal it. Something may have also come loose and in that case just tighten it up. I've had this happen several times. Nothing has died but the fish were stressed by it.
I had the same problem.
I looked at the protein skimmer and it was pumping water gack right over my return pump, creating the bubbles. I don't think it can hurt any, but it is not the best look for the tank. Aside from that, no big deal.
Ciao,
Felipe www.Felipe-Bastos.com