Have noticed that when I close the main drain, so only skimmer is sending water to pump, there is a constant presence of tiny air bubbles coming from the return that is at the deep end, nearest the pump/filter. When the skimmer is closed and only the main drain is sending water to pump, there are no air bubbles. I've a SWG, but these are not the same bubbles as produced by it. I replaced all the O rings on filter, etc. No visible signs of leaks; no water on cement pad that pump & filter sit on; no moisture on SWG or any other connections. I'm at complete loss. Anyone have any ideas?