Protein skimmers are often a good choice for keeping your saltwater aquarium clean. Besides primary biological filtration, foam fractionating (better known as protein skimming) is the most important aspect of any healthy marine system.
Although there are systems that claim to be "skimmer-free," for most of us, dissolved organic compounds (DOC), phenol oils, and other yellowing agents are a nuisance. Only active protein skimming can eliminate the need for these.
In general, all skimmers work in the same way, but there are different designs that have developed over the years. These include co-current, counter-current, venturi-style, and ETS skimmers. Each works in a slightly different manner.
It's also important to understand that different manufacturers put their own twist on the basic design. While your choices in a skimmer are vast, it remains important to understand their basic function.
How Do Skimmers Clean Water?
To put it simply, the air bubbles inside the skimmer's body strip the water of undesirable waste by-products. How the bubbles accomplish this is a neat trick that requires explanation.
Did you ever blow bubbles as a kid? Remember all the rainbow colors on them? Those pretty rainbow colors were the light refracting off the soap film. Just as the soap clung to the giant bubbles, so too does all the junk and other organic gunk in your aquarium water.
In skimmers, the bubbles are microscopic and the results can only be seen after they burst and deposit their "films" into the collection cup. No pretty rainbow of color here, only the vilest and nastiest looking sludge imaginable ride our skimmer's bubbles.
How this happens was discovered long ago in waste treatment plants. By injecting high volumes of air bubbles into a column of wastewater, the resulting outgoing water (effluent) was purer and much cleaner than before. This amazing process is all due to surface tension.
Surface Tension and Skimming
Surface tension is caused by the friction created when the oxygen bubble and the surrounding water interact. This friction, in turn, charges the molecules in the water.
Playing on the old law of physics that "opposites attract," the charged gunk molecules stick to the bubbles, riding them up the column of water. Once the bubbles reach the surface air, they burst and deposit their hitchhikers into a collection cup. This cup keeps the accumulated gunk from slipping back down into the water column inside the reaction chamber.
Due to the very nature of saltwater, this process is possible. Freshwater protein skimming just isn't feasible at a consumer level as the technology to make it happen simply isn't practical for the hobbyist.
Co-Current Protein Skimming
Bubble size is a fundamental ingredient to a successful protein skimmer and various methods are used to create the "perfect" bubble.
European hobbyists were among the first to recognize the importance of skimming their aquariums. More specifically, Germans have been responsible for designing some of the finest models. Tunze and others brought protein skimming to U.S. shores with the original design, which was called co-current skimming.
Originally, limewood was used to create the froth required in skimming and it is still employed today.
The basic co-current skimmers used an open-ended tube or cylinder with the bubble source mounted at the base. As with uplift tubes used in under-gravel filter plates, co-current skimmers use the volume of air bubbles rising in the column to bring them into contact with the system water within the chamber body. The water is "drawn" up into the cylinder from below the water's surface and once the bubbles burst at the collection cup, the treated or stripped waters simply "falls" back down into the aquarium.
Co-current skimmer designs can either be hang-on or sump-mounted.
The co-current method works but it isn't terribly efficient. The problem is what we call "dwell time," or the length of time the water is in contact with the bubbles. By lengthening the reaction chamber, more water could be processed and more gunk removed. The trouble was that not many people wanted a 6-foot tube sticking up behind their aquariums.
Research and development created the next step in skimmer evolution: counter-current skimming. You can liken this advancement to astronomy and the difference between a Newtonian telescope and a refracting telescope. Just as bending light waves by reflecting them off a mirror can double the focal length of a telescope, so too can we double the dwell time in a skimmer.
In a counter-current skimmer, the water is injected at the top of the reaction tube. The bubble source and the isolated outlet fitting are located at the bottom of the chamber. The water, therefore, has to pass against, or "counter," to the rising wall of bubbles. This effectively doubles the dwell time making for a more productive unit.
Many companies today market variations on this counter-current design.
In the pursuit of building a "better mousetrap," The Mazzei Injector Company developed what came to be known as the Mazzei valve. Today, all skimmers that use this method of air-injection are called venturi-style skimmers.
These models do not use an airstone or limewood diffuser to create the bubble column. Instead, they rely on a venturi valve to deliver both the water to be treated and the billions of microscopic bubbles. This is accomplished within the wasp-waist design.
How Does the Venturi Valve Work?
Venturi valves are easily recognizable and follow the same basic design. The high-velocity water entering from the left is bottle-necked at the molded wasp waist. The intake nipple is arranged at the top of the tube where the water movement creates air-draw, which is how bubbles are formed inside the valve. The froth exiting the valve is introduced into the main skimmer body where it removes organics.
By offsetting the fitting at the bottom of the cylinder, a vortex is created and the dwell time is magnified significantly.
For years, this was the professional's choice for serious foam fractionating, and in many circles, it remains as such. These skimmers require an outlet pipe as the volume of water that they can process in an hour necessitates a "flow-through" design. Usually, the effluent is high on the skimmer's main body, being directed back into a sump or display tank.
You can modify a common powerhead to provide virtually the same results as the venturi valve. These modifications make small volume powerheads available for smaller skimmers in micro reef systems.
You will also find that many hang-on style skimmers use the modified powerhead as the main pump. They mimic the venturi valve concept by allowing air to be drawn into the impeller housing. The impeller chops the water-air mixture and shoots it into the skimmer. It's actually quite simple and elegant.
ETS's and Down-Draft Skimming
Another and even simpler design became popular in the mid-2000s when the ETS (Environmental Tower Skimmer) was introduced to the hobbyist. Also known as down-draft skimmers, these designs can process huge volumes of water and are favored by big tank owners.
ETS models use a long tube connected to a sump with nothing more than an internal baffle plate and a drain valve. Bio-balls are placed inside the tube to diffuse the high-velocity water that is injected through the top. As the water shoots down over the bio-balls, it is smashed multiple times on the tower of bio-balls.
By the time the water reaches the sump at its base, the water is a white sea of foam. The baffle inside the sump creates dwell time. It also allows the protein-rich froth to rise up into a wide-mouthed tube with the collection cup mounted above it.
Smaller designs that follow the same principles allow smaller capacity systems to benefit as well. As with most basic protein skimmer models, individual companies offer variations on the original design.