I agree with Kevin.
Also, a pinhole leak in the plumbing can suck air into the return line as the water rushes past. Usually creates micro bubbles. Check all connections to make sure they are tight and/or submerged.
FYI, if you find Nitrates higher than expected, I recommend removing the sponge. It breaks down Ammonia and Nitrite into Nitrate but cannot do anything about breaking down the Nitrate. The bottom of the sand bed and the interior of LR are the places where Nitrate is broken down. Oh, a small growth of Caulerpa also helps eats Nitrate, adds color and movement.
Aloha,
Mark