Just to add to what Graeme said, skimmers release bubbles for many reasons, it is not always a technical reason. If the water is gas saturated either by a high level of photosynthetic activity or high KH/CO2 levels the bubbles appear to come from the skimmer but it is actually just that the water exiting is now supersaturated with gas and is degassing. Surfactants in the water like colloidal additives and certain supplements can also increase the distance bubbles carry so more bubbles exit the skimmer. I agree with Graeme, in most situations after a couple weeks of run in the Tunze 9010 does not emit any appreciable amount of microbubbles though a few may exit on occasion.