Those things are a pain in the butt. I got one for occasional filtration and carbon too. Big waste of money.
You have to be really careful to pre-fill the canister to the point of overflowing before putting on the lid. They are supposed to be able to purge themselves of air, but they don't seem to do a good job, and they shoot bubbles forever.
Also, make sure the inlet/outlet fittings are tight. They are on the loose side and the water flow through them sucks air. I ended up using RTV on mine.