In spite of what some aquarium product manufacturers would like you to believe, protein skimming ain't rocket science. Protein skimming is simply using air bubbles to collect proteins in aquarium water and bring them to the water's surface in a container.
Protein skimming (or foam fractionating) is accomplished by generating a mass of air bubbles in a water column, which collect proteins on their surface, then rise to the surface of the water. The protein is then "skimmed" off into a collection cup. The more bubbles created and the longer they stay in the skimmer water column, the more proteins can attach to the bubbles.
Different methods are incorporated to generate bubbles and keep them in the water column longer. Air bubbles want to rise to the surface of the water. If they are generated at the bottom of the skimmer, they will rise to the surface and burst, leaving the proteins. This is called "co-current" (with the water current generated by the rising bubbles) skimming.
Counter-current (opposite the current) skimmers redirect the water in a skimmer downward or sideways to keep the bubbles suspended in the water column longer.
Air is introduced into the skimmer either mechanically ("needle wheel" or venturi), or with an air pump and diffuser (air stone). Smaller bubbles have a tendency to rise less quickly, so they can gather more proteins before they finally get to the surface. To take advantage of this piece of physics, some skimmers use needle wheels or pin wheels, which chop large bubbles into smaller, finer bubbles.
Some protein skimmers, such as the Red Sea C-Skim 1200 use one pump to power the pin or needle wheel and another to move water from the tank or sump, through the skimmer, then back to either the tank or sump.
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Co-Current Protein Skimmers
Co-Current (with the water flow) protein skimmers were the first to be invented. Their design was simple and worked, but better skimmers were on their way. The Prizm Protein Skimmer is a rarity in the aquarium trade with its use of both co-current and counter current water flow.
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Counter Current Protein Skimmers
Counter Current (water flows in more than one direction) rapidly followed the co-current skimmers. Nearly all protein skimmers on the market today use some form of the counter current design. The Prizm Protein Skimmer is somewhat of a rarity in the field, using both co-current and counter current water movement.
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Prizm Protein Skimmer
The Prizm Protein Skimmer is an example of a skimmer which uses both Co-Current and Counter-Current water flow.
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Air Driven Counter Current
The air driven counter current Lee's Skimmer operates much like the updraft in a Under Gravel Filter, using rising bubble in a tube to move water upward. Small bubbles, generated by an airstone at the bottom of the skimmer, rise to the surface. Larger bubbles are injected into a small tube in the skimmer body drawing water from the bottom of the skimmer upward and back into the tank or sump. This causes a downward (counter) current. This is an example of one of the earliest counter current skimmers.Continue to 5 of 8 below.
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Needle or Pin Wheels
Needle or Pin wheels are used to "chop" the water which already contains air bubbles. This multiplies the number of bubbles as well as the surface area of the bubbles.
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A venturi is used in many skimmers to draw air into the water flow. As water is pumped through the venturi, air is drawn into the passing water through a small inlet tube. A number of powerheads use a modified version of a venturi in their outlet tube.
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Two Pump Skimmers
Some skimmers, such as the Red Sea C-Skim 1200 pictured above, use 2 pumps, one for pumping water through the skimmer (blue arrows), one for inducing air into the water with a needle wheel (white arrows).
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What to Look For in a Protein Skimmer
While the concepts of protein skimming may not be all that complicated, the number of different protein skimmers available on the market may confuse the average aquarist. What should you look for in a skimmer for your system?
First of all, the skimmer must work for your aquarium system's design. Skimmers can be placed In or On the Tank, or In or On the Sump. Deciding where your skimmer will be placed will help eliminate some possibilities.
Skimmers that allow you to adjust the water flow in the skimmer's reaction chamber make fine tuning the skimmer output much easier. Skimmers that advertise that no adjustments are needed usually don't work well for all tanks.
Ease of maintenance is very important. The easier something is to do, the more often you will do it. Your primary maintenance function for a skimmer is emptying the collection cup, so look closely at how easy it is to remove and replace the cup.