I had this issue a while back with mine. Here are the things to check, make sure all your o rings are in good condition. Give them a good coating of plumbers grease to keep them moist. The hose clamps on the valve side are tight. The other end of the hose that is in the tank, be sure that the rubber boots are completely submerged. And the most common reason that it will spit out bubbles is either your filter media is clogged or you have to much media in the trays that it is restrictions the flow of water enough to cause pump cavitation. Not damaging cavitation but enough to create the bubbles that you are seeing.
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