Considering all of the equipment that has come on the market over the past two decades, the only piece of equipment that remains indispensable is the foam fractionator or protein skimmer. Its use has revolutionized the keeping of marine organisms, especially corals and other marine invertebrates. It is called a protein skimmer, due to proteins being one of the compounds, although not the only compound removed by this device. Many different means of skimming the water have been developed over the years and each of these has positives and negatives to their use. Protein skimming utilizes the hydrophilic (water loving) and hydrophobic (water hating) aspects of various compounds in saltwater to remove unwanted compounds from the marine tank. Regardless of which type of skimmer is employed, they all work by producing air bubbles and having the water from the tank come into contact with these bubbles. When these bubbles come into contact with saltwater, electrochemical factors come into play that cause surface active compounds to adhere to these bubbles and be removed in the foam. These compounds not only include proteins and amino acids but also include molecules containing copper, magnesium, calcium and manganese as well as detritus, phenols, and microalgae (Wilkens, 1973). As you can see not all of the compounds removed by skimming are bad. Therefore when adequate skimming is employed trace elements need to be added either through water changes or trace element supplementation (Nilsen, 1990). This aspect of protein skimming will be discussed below.

The protein skimmers used in most systems are larger and more efficient than those used in the early days of the hobby. They are no longer the small two-inch diameter one-foot tall skimmers that sat inside the tank. The skimmers used today are often three to six inches in diameter and up to six feet tall. The flow through these skimmers can range from several hundred gallons per hour to over a thousand gallons per hour. The skimmers now being utilized are usually one of three designs: counter-current, venturi, or down-draft. Each of these designs has various positive and negative aspects to their use. To date there is no perfect protein skimmer, however in terms of cost, effectiveness and simplicity, the large counter-current design is probably the best design to begin with, especially for a fish only system without a large bioload. This model utilizes a wooden airstone to produce fine air bubbles inside the contact tube. This contact tube is the body of the skimmer through which all water passes. Air bubbles are produced in the airstone by a large air pump. As the air is released from the airstone the bubbles produced from the stone move up across water that is moving down. That is why these types of skimmers are called counter-current. When these bubbles break the water's surface they burst the compounds that have adhered to them are released, creating a foamy layer on the surface of the water. This foamy layer is then pushed up a tube, which is smaller than the contact tube and sits on top of the contact tube. As the foam accumulates eventually it pushes up over this collection tube and comes to rest in the collection cup where it can be readily removed. The efficiency of this skimmer is a function of bubble size and contact time. Smaller bubbles have a greater surface area than do larger bubbles and consequently more compounds can adhere to them relative to larger bubbles. Also, the longer the contact-time between these air bubbles and the water, the more compounds that can adhere. Smaller bubbles also flow through the water more slowly than do larger bubbles and therefore this increases the contact time (Delbeek, 1991).

The venturi driven skimmer produces its air bubbles by utilizing a venturi valve. When water is forced through this narrow opening called a venturi valve it draws in air and compresses it, once the air passes through this valve it expands creating bubbles. As these bubbles rise they create foam in a manner similar to the counter-current skimmer. The length of time that the water is in contact with these air bubbles varies greatly in different models of venturi skimmers, however as with the counter-current skimmers, the longer the contact time and the smaller the bubbles, the more efficient the skimmer. There are also now on the market several newer variations of the venturi skimmer. One of these is a recirculating design where the water within the skimmer passes through the venturi several times before it flows back in to the tank. As a result the water has several opportunities to come into contact with bubbles before flowing back so theoretically more dissolved material can be removed. In another variation the impeller on the pump for the venturi is of a special design so it acts to break up the bubbles into even smaller bubbles in an attempt to make the skimmer even more efficient. In both of these types of skimmers, the amount of skimmate is much greater than is typically seen in older venturi skimmers.

Recently a new type of skimmer has come on the market, called a down-draft skimmer, which utilizes a new methodology for producing foam. A simplified explanation of how this skimmer operates is that it utilizes a large water pump to pump water to the top of a long thin tube. As this water move rapidly down the tube it pulls in air, which is then released as this water comes to rest. Because a large air/water interface is created as this water moves down, the surface-active compounds that are present in the foam that is produced have time to coalesce. This foam is then forced up a collection tube similar to that found in a counter-current skimmer. This type of skimmer like the other two types can produce a lot of scum, however it has yet to be proven that this or any other type of skimmer is either more efficient or superior to the other types.

When a skimmer is functioning properly, a large percentage of dissolved organics are removed from the aquarium. This is essential in that this allows for the bacteria present on the live rock to break down the small amount of nitrogenous compounds that remain. This reduces the likelihood that any unwanted nitrogen based compounds will accumulate and produce problems.

Counter-current skimmers have been found to outperform venturi skimmers by a large degree (Wilkens, 1973). However, this was in older venturi skimmers before some of the latest advancements have been made. The reason for this was that venturi skimmers produced too much agitation in the head portion of the chamber where foam developed. This agitation does not allow for protein to accumulate as it does in a counter-current skimmer due to the constant strong motion within it. As a result in these older venturi skimmers the resultant effluent in the scum cup did not appear to be as thick and smelly as the foam collected in the scum cup of a counter current skimmer. This large amount of contact time between the tank's water and the air in the counter-current skimmer column also has the added advantage of dramatically increasing the dissolved oxygen levels within the tank. This is also now occurs in the recirculating venturi skimmers as well. The water which flows to the skimmers should be skimmed from the surface in order to maximize the amount of dissolved organics removed from the system. This is because water at the surface contains many more dissolved organics than does the water beneath the surface (Nilsen, 1990). Prior to reaching the skimmers, the water should either be mechanically pre-filtered or the detritus allowed to settle out in order to further enhance the efficiency of the skimmers. If a pre-filter is employed it should be easy to clean and should be cleaned or replaced at least once a week, as this filter should only be used to remove detritus and not to act as a biological filter where mineralization can occur.

After flowing through the mechanical filter, the water should then flow through the skimmers through gravity or it can flow into a sump from which it can be pumped into the skimmers. A flow rate of three to four times the tank's volume per hour seems to be the optimum flow rate through the skimmer although higher amounts are the norm in down-draft skimmers or large venturi skimmers. This seems to be the optimum range and if a large number of inhabitants are in the tank it may be better to run an even greater volume of water through the skimmers. However if the tank load is low in terms of animals, it is possible to overskim the tank if too much water is run through the skimmers. Overskimming means taking an excess of good trace elements out of the water, which can be just as deleterious to the inhabitants as allowing an excess of dissolved organics to remain in the water. When a properly designed skimmer is used, if the amount of dissolved organics drops below a certain threshold then the amount of foam will be reduced and no additional skimming will result, therefore it is usually difficult to overskim an aquarium. However, with the newer more efficient skimmers, it is now much easier to overskim a tank since these skimmers are indiscriminate in terms of what they remove.

The possibility of overskimming a tank has led to the debate of whether these skimmers and trace elements should be used at all since all that is really occurring is the removal of the trace elements that were just added. The problem with this argument is that it neglects the need to remove waste and other deleterious materials that accumulate regardless of whether good compounds are removed or not. It is probably best to remove as much material, both good and bad, as efficiently as possible and then add the compounds and elements that have been deemed good or positive. In this way efficient skimming can be used to maintain more positive compounds in the water by allowing what is essential to be added to replace what has been removed. This may also be another reason why it may be necessary to "feed" the corals and other invertebrates in a tank using a powerful skimmer where that was not the case when less efficient skimmers were being used. The optimal level of skimming is achieved when the skimmate collected in the scum cup is a deep chocolate brown color and both viscous and odiferous. If the scum is light yellow and thin, like a weak tea, this indicates that too much water is being skimmed and the water column needs to be lowered or the scum column needs to be raised. The scum cup needs to be cleaned regularly, usually every two to three days, it should be emptied and the skimmer itself should be cleaned of scum at least every two weeks. The airstones for a counter-current skimmer should be changed once a month while the venturi valve on a venturi skimmer should be cleaned every two months, particularly when it is smaller than one inch in diameter. The entire skimmer should be taken apart every six months and either cleaned and soaked in a weak acid solution or with dish detergent. If the latter is chosen, then unit should then be thoroughly rinsed before being replaced in a tank. The detergent should be used when the skimmer is no longer producing adequate foam or the foam no longer rises up the collection tube. This is indicative of inadequate surface tension being present due to the oily deposits having accumulated on the riser tube. By cleaning the skimmer to the "squeaky-clean" level, the surface tension can be raised and efficient skimming will recommence.

Having an efficient protein skimmer has enhanced our ability to maintain corals and invertebrates that were thought to be impossible to keep only a short while ago. Smaller and more efficient skimmers are now available in a wide variety of sizes so that they are available for just about any size of tank. Having these devices so readily available now makes it possible for just about every hobbyist to keep the nutrient levels in their tank at the same low levels as those seen in a natural reef. This will further increase the likelihood of long term success in the hobby.

Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.