Along with the use of live rock, protein skimming or foam fractionation, which is the proper technical term, has revolutionized the keeping of marine animals. The underlying principle is quite simple in that all that is really necessary is for water to pass along a column of air. This process utilizes the unique properties of saltwater, including the ionic components that result in hydrophilic (water loving) and hydrophobic (water hating) compounds in saltwater. Regardless of which type of skimmer is employed, and there are at least five different designs available, 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). Protein skimming is indiscriminate in that it removes bad compounds, dissolved organics, as well as good compounds, trace elements. Therefore when adequate skimming is employed trace elements may need to be added either through water changes or trace element supplementation.
The most commonly used protein skimmer designs include counter-current design for small tanks. The down-draft or modified down-draft design includes models by Precision Marine and the Aqua-C Innovative series for medium to large tanks. The most common designs, however, are the venturi or recirculating-venturi or needle-wheel designs of skimmers for medium to large tanks. These designs vary from the large Euro-Reef skimmers that fit in the sump to skimmers designed by Red Sea, Aqua Medic and Kent Marine that can be placed either inside or beside the sump. In addition there are now venturi skimmers that can hang on the back of the tank such as the models by Aqua-C and Prizm when no sump is being used. There is also even an efficient venturi skimmer by Tunze that sits inside the tank and rests at the top of the tank. With so many choices of skimmers for every application in every price range, there is no longer any excuse for not having one.
All skimmers have basically the same design. Air bubbles are produced either through an airstone, a venturi valve, or by water being forced downward via pressure. As these bubbles move up water is forced downward against them. During this period the hydrophilic portion of compounds adhere to the surface of the bubbles. At the top of the chamber the bubbles burst and the compounds that have adhered to them are released, creating a foamy layer on the surface of the chamber. This foamy layer is then pushed up a small tube that sits on top of the reaction chamber. 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. Also the longer the contact time between these air bubbles and the water, the greater the number of compounds that can adhere to them. Smaller bubbles also flow through the water more slowly than do larger bubbles and therefore increase the contact time.
Regardless of which protein skimmer is chosen, several factors should be taken into account, with cost being the least of them. First select a skimmer that is recommended for your size of tank or slightly larger. It is better to overskim a tank than to underskim it. Secondly, make sure that the skimmer is easy to clean and maintain. The easier this is to do, the more likely it is to get done. Thirdly, do regular maintenance on your skimmer. A skimmer only functions properly when it is well maintained. Cost should not the deciding factor because in most instances the best skimmers generally cost a little more, but the dividends of having an efficient, easy-to-clean skimmer pay for itself in the long run.
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.