I'm assuming it's new? Sometimes they need to break in a bit.
Also, make sure there's no source for the bubbles. Do you have an airstone near the intake where it could be ingesting bubbles of air?
My newest XP1 blew bubbles like you're mentioning for a few days, which aggrevated me to no end, but now it's clear, except after any servicing or resetting the unit it will blow bubbles for a few hours.
YOu can also open it and make sure the o-ring that runs around the lid is seated in the groove properly. There may be a small gap somewhere that it's getting air from as well.
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