If you have just put water in the tank, then the air comes from the action of the water pouring into the tank. If it is established, I would check to make sure no air is getting sucked into the return pump which creates microbubbles.
My new tank started sucking air and I ended up with thousands of bubbles on the glass and in the rock crevices. Once I adjusted the flow and the air stopped going into the pump I wiped away the bubbles on the glass with my glass scraper and used a small powerhead to force the bubbles I could see (and a lot I couldnt) from the rock.
Why do I love fish? Because every time I stare into the little world I have created for them, every problem I have just floats away....