How far below the surface is your output nozzle? Mine is about 2 and 1/2 inches but any higher than that and I get bubbles from to much turbulance on the surface.
You may also have alot of air trapped in your canister. Turn it off, Make sure the canister is filled nearly to the top with water, Plug everything in, turn it on for 15 seconds, turn it off. Then tilt the canister to allow excess bubbles to escape up the hoses.
Repeat this process of turning on for 15 seconds and then turning off and tilting until very few bubbles are returning up the hoses. That Outa do ya!