A skimmer is one of the most important pieces of equipment when starting a new tank. A skimmer serves two very important roles in a reef tank. It's primary function is the removal of dissolved organics (waste) from the water column before the waste can break down into ammonia, nitrite, and nitrate. This breakdown of waste is known as the Nitrogen Cycle and leads to nitrate build up in your water.

The second function of a skimmer to aerate the water. It is critical to have your tank operating with supersaturated oxygen levels (are darn close to it). At night when photosynthesis stops (lights out) everything starts to take in oxygen and expel carbon dioxide. This build up in carbon dioxide will lower your pH levels and keep chipping away at your alkalinity levels (buffering capacity). If a skimmer pump was to stop working on a tank with a heavy bio-load, it is possible that the tank can get wiped out from low oxygen levels. A tank will run fine for a while with a build up of dissolved organics, but critically low oxygen levels will be catastrophic.

The key to a skimmers functionality is a combination of air-bubble size and contact time. Ideally, you want the smallest bubble size and the longest contact time possible. As odd as it sounds, the smaller the air bubbles the more surface area you are creating for dissolved organics to stick to. The low surface tension of saltwater allows very small micro-bubbles to be created. In fact, skimmers are pretty useless for waste removal in fresh water systems because the surface tension of freshwater does not allow you to create micro-bubbles.

Contact time is the term used to describe how long a bubble remains inside the skimmer swirling around attracting dissolved organics before reaching the top of the skimmer. When the bubble reaches the top of the skimmer it generally pops or becomes part of the foam column at the top of the skimmer. As the wastes gets concentrated in the foam column, the foam gets thicker. This thickness traps air which pushes the thick foam out of the skimmer. The foam then falls into a collection cup. A skimmer is unique in this way as it is the only method for exporting waste from the system. Other filters such as mechanical filters trap waste or convert waste, but do not remove them from the water column unless someone actually removes the filter medium and replaces it. But between replacements trapped debris breaks down and enters the nitrogen cycle.

Different types of skimmers make different kinds of waste. Many smaller skimmers really don't make a thick foam commonly called a "dry foam". Instead they make a "wet foam" which looks more like water (called skimmate). This water should never be clear, but can range from brownish, to greenish. Which is better? Who knows. I like a more concentrated, dark liquid. More efficient skimmers make a dryer foam which looks like ugly, stinky, bubble bath foam. Almost like a paste. The skimmate should also smell bad, real bad. I mean a small sniff should make your noise hairs curl up and shrivel. Any less, then you don't have a truly efficient skimmer on your tank.

It's critical that you clean your skimmer frequently. How frequently? That depends on how dirty your water is. A good rule of thumb would be, if you can't see inside it, its time to clean it. The gunk that builds up on the walls of the skimmer pop the air bubbles to early and reduce the efficiency of the skimmers output. I use a new toothbrush or baby bottle brush and a wet sponge to clean out the insides of my skimmers.

Some advanced reefers are able to run reef tanks without skimmers. This should not be attempted by a novice, and should only be tried on mature tanks (say about 2 years old). If you do try to run a tank skimmerless, you just don't shut it off. You need a strategy to deal with lower oxygen levels and other forms of waste removal.

By Richard J. Durso, November 1998.