Coralife skimmers are notorious for micro bubbles. The best way to get rid of them is to put a big piece of sponge on the return end. If it is a fairly new one, it should have one already. If not, some of the models had a box on the return. Adding a piece of sponge in it helps as well. The only other reason my old one would do that was if I ever dosed meds or even certain additives would cause it to go crazy until I did a couple water changes. Hope this helps.