While we are all striving to replica nature and work towards more natural settings for our watery pets, I still strongly advocate the use of protein skimming to help maintain proper water quality for our aquariums. Some may find this statement at odds (the word "natural", with a man-made device such as the protein skimmer), but protein skimming is a perfectly complimentary strategy for replicating a process of the natural world in the entirely unnatural confines of an aquarium. Specifically, the dynamic process in either environment exports significant amounts of organic waste (proteinaceous matter attracted to the bubbles in a skimmer or carried to shore from a reef as sea foam) before it can degrade in the aquatic environment. This is not to say that you cannot succeed without a protein skimmer, but it is my belief (and experience has demonstrated time and time again) that more people can and will benefit enormously from employing this technology than those who choose not to employ one. For newer aquarists and systems with heavy bio-loads, a protein skimmer should be considered an indispensable piece of hardware as well as a fundamental investment in a successful aquarium. In a nutshell, if you cannot explain why it is that you don't need a skimmer, then you do need a skimmer! Finding a good and affordable unit is the real challenge.
Protein skimming, also known as "foam fractionation", has its roots well established in industrial applications. It has been employed in waste treatment since the 1890's to separate various metals, proteins and surfactants from solution and has been used in marine aquarium keeping at least since the 1960's. How does it work? Simply stated, a protein skimmer mixes tiny air bubbles (consistently produced and sized) with water in a contact chamber in an effort to separate dissolved organics from the main body of water. This separation of organics is achieved by the polar properties of dissolved organic molecules and their attraction to the "charged" air-water interface of fine bubbles. These large, polar protein molecules have a positive and negative end and when they come into contact with small bubbles, they end up coating the surface, much like soap does. It is interesting to note that the same forces that attract these compounds to the air bubbles in your skimmer also attract such elements to the air-water interface at the surface of the aquarium, hence the need for very efficient collection of surface-skimmed water to feed your protein skimmer.
Dissolved organics are channeled inside the contact chamber of the protein skimmer with foam and bubbles where they migrate upwards into a collection cup. This is the crux of protein skimming. It is the only means of filtration that actually exports organics entirely from the water. Other means of physical, chemical, and vegetable filtration merely breakdown or absorb/adsorb compounds, but they still allow such elements (proteins, scatols, phenols, metals, etc.) to remain in the water column. They are just in a different form or location, like plant or algae tissue, by having been taken up as nutrients. Here, they can easily be imparted as degraded byproducts or contaminants wholly back into the aquarium system. Skimmate, however, is completely exported and isolated from the system water.
While, it is true that protein skimming removes more than just undesirable elements from the aquarium. It will also remove some useful compounds, too. This is one of the reasons why some hobbyists resist their usage. However, by the same token, if we want to exclude everything in the aquarium that takes out desirable organics and bio-minerals, we would have to remove our corals and other invertebrates too, and the fishes, desirable algae, true vascular plants, etc. You get the drift! Without a skimmer, the water will still be scrubbed of desirable components. In my opinion, protein skimming far and away serves the greater good. Over time, we are beginning to understand the nature and quality of noxious compounds that are being produced in our systems. Protein skimming helps to reduce the amount and impact of these "toxins".
How much skimming is enough?
I can assure you that a well-designed skimmer, when properly sized and installed, is quite capable of removing a full cup of dark skimmate daily. When you see and smell this product you will be hard pressed to find another hobbyist that would pour the disgusting, thick, foul-smelling mess back into an aquarium. Now imagine being able to extract that garbage daily (or leave it behind to concentrate and feed nuisance algae!) and then you will see things from my point of view. While some specialists and researching aquarists may want to reduce or decline the use of a skimmer for deliberate reasons, casual aquarists will significantly benefit from daily protein skimming. Intermittent skimming is also a rather dubious affair, in my opinion. If you cannot say precisely what it is that you are trying to avoid removing in doing so, or do not test to monitor how well you are succeeding at it, my advice would be to commit to full-time skimming. Specialized aquaria with a heavy population of filter feeders and tanks dominated by aposymbiotic species may be some of the few systems where reduced or periodic skimming may be in order. Even in such cases, however, the heavy feedings required may still warrant the support of a good protein skimmer to maintain good water quality. It is hard to avoid using a skimmer in the modern marine aquarium.
I would also like to debunk the legend in popular literature that protein skimmers remove some desirable plankton. This is mostly untrue and insignificant to any measurable degree. As stated above and beyond, the process in fact works on a molecular level (polar attraction of dissolved organics to the air-water interface of tiny bubbles). Plankton is simply too large and not attracted (literally) to the process. There has also been some concern about protein skimmer pumps (like most pumps) harming plankton by "impellor shear". While this is true to some small extent, most plankton passes through easily, unharmed, and all is still edible by fish and coral on the other side of the pump! At a time when we are seeing hobbyists employ high flow, direct-drive pumps (return and closed loop strategies) with circulation of the aquarium approaching a 20 times turnover per hour or more, to be concerned about a protein skimmer running at 3 to 5 times a tanks total volume seems insignificant at best. The merits of protein skimming far outweigh the drawbacks.
The selection of a skimmer is both a personal and subjective choice. You need to evaluate whether a given skimmer will work at all and then you need to decide how much work you are willing to put into getting it to perform optimally. Some of these objective concerns are:
? Contact time (how long the bubbles are in contact with the solution)
? The size and consistency of the bubbles produced (generally smaller is better, in this case)
? The volume of bubbles produced
? The ratio of air to water
? The total volume of water processed
? And the impedance or turbulence inside the reaction chamber (the interruption of protein carrying bubbles is to be minimized by chamber design)
Some of the subjective concerns include:
? Ease of installation
? How much adjustment does it require? (Well designed units require very little maintenance to perform consistently)
? How easy is it to clean? (Remember you are doing this several times weekly if not daily)
? And lastly, how much does this thing cost? ("Good things are seldom cheap and cheap things are seldom good" as the saying goes, but you do not need the biggest and most expensive unit either.)
Incorporating a Protein Skimmer
A myriad of protein skimmer styles are discussed below, with some of these in turn being available in one of two versions: hang-on and in-sump models. Hang-on skimmers merely cling to the back of the aquarium much like the power filters that most aquarists are familiar with; and in-sump models are, of course, located in a lower (downstream) reservoir or sump. Regardless of which version you choose among any of the styles below, there are a few important guidelines to heed to enhance any skimmer's performance. As mentioned above, the components that we are trying to remove are attracted to the surface (interface) of the aquarium. As such, all skimmers should be fed (pumped or overflowing) at least some raw water from the tank's very surface. Some hang-on skimmers come with surface extraction boxes, which forces the skimmer to draw water from the top of the aquarium. Sump model skimmers generally receive their water from built-in overflow boxes, bulkhead fittings, or other methods of draining surface water to the sump. Some well-designed sump models, however, still will not work properly if they are simply placed in an open sump with a fluctuating water level. Beyond the slight variations in head pressure on pumps due to the fluctuating sump levels, the concentrated overflowing water pools and can separate proteins again within a large reservoir and avoid the submerged feed pump for the skimmer. Sump model skimmers should be placed in a "catch-basin" (a contained, constant level 1st stage partition or a "sump within a sump") to collect all raw water from above before flowing downstream. This vessel need not be much bigger than the footprint of the skimmer to keep the protein rich water concentrated in the skimmer/pump well. It is also a good idea whenever possible to employ a series of baffles in a modified sump to restrict protein rich surface water inside the skimmer section while allowing excess water to flow into the rest of the sump. Furthermore, any pre-filter or other mechanical filtration between the surface-collected water and the skimmer is a serious impediment to the unit's performance (like overflow box pre-filters which severely compromise skimmate collection or the placement of a skimmer after a filtration component like a refugium or trickle filter). It cannot be overstated; the best producing skimmers receive raw water from the very thinnest layer of surface water. In concert, these two critical limitations, the need for a static water level in the tank and/or sump and the focus on the delivery of surface-skimmed water, bring us to look very closely at the placement of water pumps on pump-fed skimmers.
Other factors to consider about optimizing skimmer performance include:
? Keeping venturis cleaned (purged regularly with demineralized water and twice yearly given a vinegar bath) or changing wood airstones (where used) frequently (4 week maximum) to insure fine and consistent bubble size.
? Recognizing that sharp changes in barometric pressure effect bubble size and skimmer performance: do not mis-adjust hardware impetuously, but be patient and wait for "normal" pressure and performance to resume.
? Low salinity or erratic changes in specific gravity (as with sudden influxes of freshwater for evaporation top off) limit skimmate performance.
? Frequently putting one's hands in the tank is disturbing for many sensitive reef invertebrates, but is especially disruptive to skimmer production as anything that breaks the surface tension is a compromise to the collection and concentration of proteins. Keep your hands out of the aquarium as much as possible!
? Try to reduce the number of variables that affect your skimmers performance so that you can control skimmate production. Beyond the above listed suggestions, recognize that there are two fundamental variables in a skimmer: water flow and airflow. Work very hard to keep one variable constant while using the other to tweak skimmate performance. If your skimmer is producing too much wet foam (watery), then reduce air or water flow slightly. If you get very little or no dry foam, then increase water or airflow slightly.
In gross terms (and in every definition of the word gross), ideal skimmate should be darker than light, and more thick than thin. We're looking for "strong coffee", not "weak iced tea" here.
I now turn our focus to skimmer styles...
Air-Driven skimmers were the first models to enter the hobby in co-current (air flowing with the direction of feed water: a poor design for aquariums) and counter-current styles (air flowing against the direction of feed water: a better design for aquariums). They may be internal or external in nature, and gravity fed or pump fed. These simple units use air bubbles produced by strong air pumps and generally wooden air stones. Even though they are called air-driven, some come with a water pump that feeds the skimmer raw water to be processed. Most modern incarnations of this style are a counter-current design. Essentially, this means that water enters near the top of the skimmer and exits from the bottom while tiny air bubbles are injected near the bottom and work their way to the top with resistance from the water flow which increases contact time and strips proteins from the raw water.
Counter-current air-driven protein skimmers can be very efficient at removing significant amounts of quality skimmate. They have lost favor with some hobbyists because of their dependency on sometimes frequent maintenance and adjustments. Wood air stones decay over time, which results in increased backpressure, decreased bubble production, and less skimmate extracted. I recommend the replacement of wood air stones monthly or better to maintain optimum performance. Another drawback to air-driven skimmers is the need for an air pump. They can be noisy and generally use a rubber diaphragm to pump the air. These diaphragms degrade if not rupture in time. While they last considerably longer than wood air stones, at a year or more for most, this is another aspect of maintenance and upkeep to consider.
Advantages to Air-driven skimmers:
? Generally inexpensive to buy and easy to install
? Simple adjustments and predictable skimmate production with proper installation
? Many tried and true DIY plans available abroad for the ambitious and crafty
Disadvantages to Air-driven skimmers:
? Often labor intensive with sometimes daily adjustments needed to maintain skimmate production
? Wood airstone are limiting as a natural product with variations in quality and longevity by batch
? Ongoing expense of airstones and air pump diaphragms
? Must be exceptionally tall to serve large systems
Venturi air-injection was the next big technological development in the evolution of marine aquarium protein skimmers. A venturi is a specially designed piece of plumbing with a profile that resembles an inverted "T". Water is forced through at high velocity, which causes a low-pressure suction on the open stem (top) of the inverted T and draws air in. You may have noticed this feature on powerhead water pumps. In this application, venturis are used to inject air into their water stream to aid in oxygenation. In this application they are rather crude devices, but you get the picture. Protein skimmer venturi valves, however, are high precision instruments engineered to produce a very fine-sized air bubble. The better ones also come with a threaded needle control valve for fine-tuning the amount of air injected into the skimmer.
The venturi-driven skimmers were a significant improvement over air-driven skimmers. The keystone of a good, simple venturi skimmer is the water pump that drives both the air and water flow, ideally, with great consistency. This should translate into less maintenance/adjustment (perhaps weekly instead of daily) for the aquarist and more reliable skimmate produced. In contrast, air-driven skimmers may require you to adjust both the amount of air and water fed into the contact chamber, which can complicate attempts at producing dark skimmate consistently. Venturi skimmers require that their water pump be cleaned frequently, like any water pump, of buildup (bacterial slime and mulm) on all interior parts and surfaces. You will also need to periodically clean the venturi stem of dust and debris like salt creep or scale that accumulate. Comparatively, venturi style skimmers cost a little more than air-driven models, but for that extra money, they should produce more skimmate regularly if designed well.
Advantages to Venturi skimmers:
? Well-designed models are a better value than air-driven models: a lot more production for little extra cost
? Some good designs are available and affordable for systems on a budget
? Lower maintenance and no significant expense of upkeep (no air stones or pump diaphragms)
? Most designs occupy comparatively less space: compact units are available and worthy
Disadvantages to Venturi skimmers:
? The hobby market is flooded with inferior models in this style (research your brand and prospects well!)
? A reduced contact time in compact models demands properly delivered feed water for efficient skimming (this has been addressed/corrected in some models with extended contact centrifugal chambers)
The next leap in skimmer design brought us the aspirating models. These differ from a venturi in principle by pulling air through a pump impeller on the intake side (aspiration then fractionation) instead of drawing air from the output side of a water pump via the rush of exiting water. Basically, an aspirating pump sucks in air with water, uses the impellor to "chop up" the air into finer bubbles, and then injects this foamy mixture into the contact chamber.
The price range of this style runs the gamut from some of the least through the most expensive skimmers on the market. Unfortunately, some of the cheapest ones out there also tend to be some of the worst performing skimmers. The difference, primarily, is in the size of the unit and the design of the impellor. Needle-wheel impellors are very different from the standard impellor used in some aspirating models. Modification of the impellor allows an aspirating model to efficiently blend air and water more effectively and produce smaller and more consistent bubbles. Now the question arises, why don't all aspirating skimmers use needle-wheels or perform similarly? The answer is needle-wheels are a patented technology. It costs real money to license and produce this kind of hardware. The higher end skimmers of this style are very effective, easy to adjust, easy to clean, and moderately sized (modest spatially).
Advantages to Aspirating skimmers:
? Top shelf models are categorically consistent and reliable (bubble size, skimmate production, etc.)
? Lower maintenance and no significant expense of upkeep (no air stones or pump diaphragms)
? Most occupy comparatively less space than other styles in their competitive price and efficiency range
Disadvantages to Aspirating/Needle skimmers:
? Like venturis, the market is flooded with inferior models in this style (research your brand and prospects well!)
? Even infrequent cleanings of the delicate needle-wheels are precarious. Damage to them can be expensive to repair or replace
Downdraft skimmers are some of the most powerful skimmers available and quite popular with large home aquariums. They use a big pump to inject water down a narrow tube at high velocity. This thin tube usually has bioballs or like media in it. These are not intended for biological filtration, although some bacteria will inevitably develop there and participate in biological filtration. The real purpose of this media is to whisk water through the baffles (bio-balls) at a high speed to create ultra fine bubbles like whitewater in rapids. The crashing water forced through the bio-balls creates very effective foam, which is then channeled into a box below before continuing on its way to rise and accumulate skimmate via a larger reaction chamber.
Downdraft skimmers are inherently expensive due to their large size, raw materials needed for production (thick acrylic), and the amount of skilled craftsmanship needed for fabrication. They also require a large water pump, which adds to both their initial cost and ongoing operational expenses; for many smaller tanks they are overkill with regard for efficacy and purchase price. Most units are also somewhat to very unfriendly to clean and service routinely (recessed and inaccessible regions, tedious thumb screws for disassembly, etc). On the positive side, however, they are relatively easy to adjust and perform exceptionally well on large systems with heavy bioloads.
Advantages to Downdraft skimmers:
? Consistent and reliable bubble size and skimmate production
? Well suited for large aquariums and heavy bio-loads
Disadvantages to Downdraft skimmers:
? Expensive to purchase and operate
? Difficult to clean and service
? Necessarily large and cumbersome by design
? A poor value for performance versus price paid (more "bang for your buck" on other styles)
? Noise of operation is can be an issue for some aquarists
? The large pumps required impart significant heat to system. This can be dangerous for systems already operating on a precarious threshold.
There is another style of skimmer I want to briefly mention. These are the so-called Beckett skimmers. I think of these as more of a version or better yet an evolution of the Downdraft units. They appeared soon after downdraft units. At a glance, they are very similar to downdraft models. In gross form, they have two tubes like a downdraft skimmer: one to mix air and water, and the second to work as a reaction chamber. But, unlike a downdraft skimmer, instead of relying on the high velocity crashing of water through bio-balls to form a frothy foam, they use a special nozzle, called a "Beckett" after the pond company that makes a similar nozzle for pond aeration. Beckett-style skimmers have much of the brute force of downdraft skimmers. Alas, they also come with the same drawbacks: the sheer size of both the unit itself and the pump required to run it, the cost of the unit to purchase and operate, heat imparted from large pumps to the system, noise of operation, and the cleaning hassles.
Spray Injection is one of the latest skimmer styles to come into the marketplace. These look like smaller downdraft designs, but there are some major distinctions that set this technology apart. The easiest way to envision how a spray injector works is to think about washing your car. Wait a minute! What are you talking about, you may ask? Well, bear with me for a moment. Imagine getting out your car-washing bucket (very different than your fish cleaning buckets!). Add some soap to the bucket and begin to fill it with water from the hose. If you allow the water to merely pour quietly into the bucket, you will create some large bubbles and a little bit of foam. Now, if you place your thumb over the hose to produce a high-pressure spray, the water and soap in the bucket will foam up like crazy! This is the principle behind spray injection technology.
Because of their engineered injectors, these skimmers can be smaller in size than comparable Downdraft and Beckett skimmers and use a smaller pump. In doing so, they are more user-friendly and affordable than comparable styles. They are also some of the easiest skimmers to adjust and maintain.
Advantages to spray injection skimmers:
? Vigorous and reliable bubble size and skimmate production
? Delivers performance of a larger skimmer style at a fraction of the size
Disadvantages to spray injection skimmers:
? Despite compact size and efficiency, still an expensive style in contrast to a good venturi or countercurrent
? Noise of operation is an issue for some aquarists
Protein Skimmer Summary...
? Buy the best skimmer that you can afford in the beginning.
? Select a model that is likely to require no more maintenance than you can reliably commit to. Easy to clean and adjust.
? Clean interior skimmer neck and any air-injecting apparatus frequently! Weekly or more frequently for neck and cup cleanings to improve skimmate production.
? Proper installation is crucial for success! Surface extraction and static water levels are critical. No open sump applications!
? Strive to collect a full cup of dark skimmate daily from any aquarium system. Low nutrient systems are far less common than poorly adjusted, designed, or installed skimmers.
? Never pre-filter water fed to a skimmer. All water from surface extraction to the reactor chamber should be raw, concentrated and undisturbed. Any filter pads, foam blocks or other obstructions along the way will seriously impede skimmate production!
? For large systems and those with heavy bio-loads, two skimmers cleaned alternately is a good strategy to ensure consistent nutrient export and skimmate production.
? Research your selection before buying this crucial component. Popular does not always mean effective.
There are some popular legends in the hobby about protein skimmers that generally hold true. Foremost of which is that better designed skimmers require less maintenance but don't necessarily produce better skimmate. Indeed, the bottom line at the end of each day, week and month is how much pollution/skimmate was removed. It makes little difference to the system if it takes you daily adjustments of a skimmer or once monthly adjustments to collect the same amount of skimmate: the net benefits of nutrient export are the same either way. How hard you are willing to work to get optimum production is the difference that makes the choice. The best "top shelf" skimmers afford you the luxury of less maintenance (with reliability), while the better "economy" skimmers demand that you earn your way with due diligence. Some aquarists enjoy "tinkering" and "fussing" over system hardware while others favor a more automatic approach. Either strategy can serve as a means to the same end. While I cannot name the "perfect" skimmer for you, hopefully I have offered a few suggestions and guidelines to help you make your choice. First of all, ask the opinion of your trusted local fish store. There you should be able to find knowledgeable opinions from industry professionals who can advise you on your purchase based on practical experience and knowledge of your specific needs. Secondly, seek out the opinions of experienced hobbyists. You can usually find very helpful people with intelligent insight at your local aquarium society or club, and also on some of the Internet message boards. These are often individuals who have been around and witnessed many different brands and models at large. Thirdly, remember to be mindful of the price of a skimmer in the big picture. Think of a better skimmer as but a small percentage of your total investment in the health and well being of your aquarium life. Realize that buying a cheap skimmer, finding out that it does not work, and later buying a better skimmer is more expensive than simply buying the best skimmer you can afford in the beginning. And lastly, use the above perspective as but part of your information gathering process to formulate an informed decision for your specific needs.
I would like to thank my dear friends Anthony Calfo, Adam Cesnales, and Robert Fenner for their advise, insights, and editorial content for the above piece
Calfo, Anthony. 2001. Book of Coral Propagation: Reef Gardening for Aquarists. Monroeville, PA: www.ReadingTrees.com Publications.
Calfo, Anthony. Personal Comunications.
Cesnales, Adam. (Owner of Marine Acrylics) Personal Communications.
Delbeek, J. Charles and Julian Sprung. 1994. The Reef Aquarium: Volume 1. Coconut Grove, FL: Ricordea Publishing.
Escobal, Pedro Ramon. 1996. Aquatic Systems Engineering: Devices and How They Function. Oxnard, CA: Dimension Engineering Press.
Fatherree, James. 2001. "Marine Matters: The Mysterious Aquarium Gadget: The Protein Skimmer" Tropical Fish Hobbyist. September 2001.
Fenner, Robert. 1998. The Conscientious Marine Aquarist. Shelburne, VT: Microcosm Ltd.
Fossa, Svein A. and Alf Jacob Nilsen. 1996. The Modern Coral Reef Aquarium: Volume 1. Bornheim, Germany: Birgit Schemettkamp Verlag.
Kim, Jason. (Owner of Aqua-C) Personal Communications.
Knop, Daniel. 1996. Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams. Ettlingen, Germany: Dahne Verlag GmbH.
Marini, Dr. Frank. 2002. "Skimming Basics 101: Understanding Your Skimmer". www.ReefKeeping.com June 2002.
Moe, Martin A. Jr. 1992. The Marine Aquarium Reference: Systems and Invertebrates. Plantation, FL: Green Turtle Publications.
Shimek, Dr. Ron. 1999. The Coral Reef Aquarium. New York, NY: Pearson Education Macmillan Company.
Shimek, Dr. Ron. 2002. "Down the Drain: Exports from Reef Aquaria". www.ReefKeeping.com December 2002.
Tullock, John H. 2001. Natural Reef Aquariums: Simplified Approaches to Creating Living Saltwater Microcosms. Neptune City, NJ: TFH Publications.