[Frogfish]

I would also look at turning off/on some equipment. Are the bubbles coming from the skimmer? Turn it off and see if they get better. If so you need to modify the skimmer/bubble trap. If the bubbles are coming from water falling over a baffle then you need to raise the water level downstream of the baffle.

Basically experiment to see where they are coming from. If nothing else, you could always silicone your PVC joints to ensure that there are no air leaks.