Normally, when my filters are bursting bubbles out of the outlet, there are two possible causes. The intake is restricted/blocked(mine have sponges and they may get clogged) or there's a micro crack in the plastic parts that are above the water surface and they suck air in. That happens if the joints are not good enough as well and/or the water levels goes down due to evaporation. If there's is an air leak, it's good if it's under water but causes the filter to suck air in when water goes down. And of course, make sure the inside of the filter/the filter media/impeller is clean so the pump is not having a hard time, which produces micro bubbles via the outlet as a result.
Edit:. Sorry, I see it's a pump, but exactly the same applies. It will produce air bubbles if there's an obstruction or an air leak somewhere along the line.