I'll assume it's a cannister. On my Fuval 403's before I reinstall the cannister I fill it first with treated water. After connecting the hoses I open the input first, then I open the output valve. After running the motor for a few minutes or seconds depending on the bubbles. I turn it off until the flow stops then repeat until all bubbles stop. If they keep returning then make sure the input is not picking bubbles up in the tank. In the worst case your pump impeller may be cavitating and is creating air bubbles in the pump.
Not one shred of evidence supports the notion that life is serious.