Typical microbubbles will make the water almost appear cloudy and can irritate corals. There are usually a couple suspect causes:
1.) Your sump, if you have a sump, the flow rate may be too high. One fix is to use a filter sock where your drains dump into the sump.
2.) Your skimmer: some skimmers have a tendency to release microbubbles, you can also use a sock or sponge to assist with this. Also, you can make sure the skimmer output isn't too close to your return pump.
3.) Your plumbing: This is the biggest pain but also a common cause. Tiny pin leaks can introduce air in your system an wreak havoc. The negative pressure due to the high flow rates suck air in without showing any signs of a leak. You will need to test all your fittings and check for salt creep. FYI, I battled this for a while on my new system and finally broke down and replumbed my return.
4.) Cyano and/or algae: If you are in the midst of a bloom, you can actually get microbubbles from cyano or algae. As they offgas the byproducts of photosynthesis, small bubbles are released. These can get caught up in your pumps and chopped up into finer bubbles creating a problem.
