I'm no expert but i have 3 theories, It could be Nitrogen gas. It is from anaerobes doing their job and breaking down Nitrates into free Nitrogen gas, I believe it's harmless. It's just doing it's job.
Or... Many forms of algae will create bubbles in the tank. Diatoms, cyano
, hair algae at times. Does it look like the bubbles are coming from an algae source?
Then the other issue that was alluded to could be microbubbles coming from your skimmer. These are usually tiny bubbles that are more of a nuisance to you than anything else as they can make the water look kind of cloudy.
If you don't have a skimmer then that theory is out the tank so to speak.