Protein Skimming
If you've ever visited a beach on a windy day, you may recall seeing foam washing up on shore. This foam is produced by the
churning action of the waves, combining air, the water, and certain polar organic compounds dissolved in it to form a stable foam. The wind blows the foam to shore, and thus helps to purify the water. Protein skimming, or foam fractionation as it is sometimes called, works in a similar manner, mixing fine air bubbles with water. By collecting the foam that forms, proteins and other organic compounds are removed from the aquarium water. This is very beneficial to the health of Reef Aquariums" href="/reef-aquariums/category-8.html">marine invertebrates and fish. Of the various chemical filtration methods available, only protein skimming physically removes most organics from the aquarium before they begin to break down (Moe, 1989). This reduces the load on the biological filter and raises the redox potential of the water. Protein skimmers primarily extract dissolved substances from solution, though electrostatic attraction to the bubble surfaces and physical trapping by the thick foam do also draw some particulate matter, bacteria, and phytoplankton out of the solution. The list of substances removed by skimming includes amino acids, proteins, fats, carbohydrates, phosphate, fatty acids, phenols, iodide, metals such as copper, iron, and zinc complexed with the proteins, detritus, and leachates of plant and animal origin. A skimmer thus lowers biological oxygen demand, chemical oxygen demand, and nitrate build-up (Dwivedy, 1973; Lomax, 1976). The removal of organic acids also helps to maintain the pH of the system (Dwivedy, 1973). Furthermore, since skimming removes toxic organic substances released into the wrater by the invertebrates (e.g. terpenes from soft corals), it makes it possible to keep a variety of coral species in a confined space.
There is still a great deal of debate about whether protein skimming removes a significant amount of trace elements, compared to the metabolic actions of the animals, plants, microorganisms and bacteria growing in the aquarium (Achterkamp 1986; Keith 1980). However, there is no doubt that some elements such as iodide and iron are removed both by
skimmers and natural biological processes, and that they must be replenished (see chapter 8).
Protein skimmers for aquariums have been around for decades, but seem to be catching on only recently in North America. A protein skimmer consists of a column wherein a mixture of aquarium water and very fine air bubbles come in contact, and the resulting foam that collects at the top of the column is directed out into a collection cup. The first two types of protein skimmers available to aquarists operated either by co-current flow of air and lis water, or counter-current flow. The latter mode of operation is more efficient because the water has greater contact time with the bubbles, and the bubbles collect foam in a gradient such that they have low concentration of surfactants where they enter at the
bottom of the skimmer, and high concentration at the top. The water has high concentration of surfactants where it enters at the top, and these adhere to the surfactants already accumulating on the rising bubbles as the water passes downward through the column. There are many other designs and types of skimmers, and we shall describe them shortly.
While the mechanics of protein skimming are simple to explain, the subject of protein skimming gets a little more complicated when we examine the chemical processes that make it effective. Certain molecules in the aquarium are attracted to the surface of air bubbles. They are referred to as surface-active molecules or surfactants. These organic molecules are polar at one end and non-polar at the other end. The polar end is attracted to water molecules (hydrophilic) and the non-polar end is repelled by water (hydrophobic). These surface active molecules are attracted to a surface where the hydrophobic end of the molecule can be away from water and the hydrophilic end can remain in contact with water. Air bubbles provide an excellent air-water interface for the surfactants to adhere.
In order for the skimmer to work effectively, the air bubbles should be quite small, preferably between 0.5 mm and 0.8 mm in diameter (Achterkamp, 1986; Spotte, 1979; N. Tunze, pers. comm.). This is why protein skimmers utilize an airstone or Venturi. The most common method of making these fine bubbles is to use an air stone, usually a wooden air block. However, there are a number of problems with using air driven stones or wooden blocks. First, an air pump does not supply a constant air pressure over time, so the rate of bubble formation varies considerably. Secondly, wooden air blocks are very susceptible to rotting, which blocks many of the air pores. Other types of airstones become clogged by mineral deposits. This results in a varying bubble size and rate of production, which ultimately results in a poorly functioning skimmer (Achterkamp, 1986). An alternative method to the wooden air block and air pump combination is the use of a Venturi valve. A Venturi valve consists of a small device that directs the stream of water in a pipe through a very small opening or constriction in the pipe. There is a pressure differential between the two sides of this small opening; high pressure before it, and
lower pressure after it. One or more small ports in the Venturi lead to the outside. The pressure drop after the constriction causes air to be sucked in through these openings, forming a very fine mixture of air and water, which is introduced into the bottom of the protein skimmer. In free-standing external skimmers with a Venturi located at the bottom, the air input port must have a hose attached to it that extends above the height of the skimmer to prevent water from back-flowing out of the Venturi when the power is off. A check valve system could also be employed to prevent back-flow of water, but we don't recommend check valves because they are not 100% reliable.
A Venturi rarely needs to be replaced, and circumvents many of the problems inherent in wooden air blocks, but it is not completely trouble-free either. To make the valve work properly, the water must enter under pressure. This requires that the water be pumped to the Venturi by a strong water pump. Some Venturis have a valve attached that can be adjusted to give the optimum air volume to generate proper bubble size and abundance. The valve needs to be checked and adjusted during periodic cleaning of the skimmer. There are now a number of skimmers available on the market that utilize the Venturi principle, and they are called "Venturi skimmers". Some Venturi skimmers have the pump that powers the Venturi on a closed loop, taking water from the skimmer and sending it back to the skimmer, for the purpose of packing it with a dense column of air bubbles. The Venturi reduces -the volume of the pump output, so it makes sense to use a separate pump to generate the bubbles, while gravity feed or another pump allows the passage of as much water through the skimmer as possible.
Venturi skimmers tend to be extremely sensitive to changes in their immediate environment. For example, aerosol sprays, paint fumes, or wood dust in the same room as a Venturi operated skimmer may lead to a marked decrease in foam production for a few hours. These things also affect skimming with other types of protein skimmers, but less rapidly. Placing one's hands in the aquarium, or feeding the fish, can also cause a sudden temporary reduction in foam production due to fats and oils introduced to the wrater that lower the surface tension, as we shall explain.
A recent design that is gaining popularity in Germany uses a special aspirator system made from a high rpm pump and an impeller with pegs on it. Air is drawn in on the suction side of this pump, and the
line is not constricted, in contrast with Venturi design. The advantages of this design are many. The pump does not need to be so large, as it does with the Venturi design in order to develop sufficient pressure behind the constriction of the Venturi. The volume of air drawn into the stream of water is also much greater than by Venturi. The pegs on the impeller chop the air into fine bubbles, so this design packs the skimmer with a high density of fine bubbles. It appears to be the most efficient design for mixing the most air with the water. In the skimmers we have seen using this technique, the aspirator pump merely generates air on a closed loop, drawing water from the skimmer and sending it back to the skimmer. The air water mixture is injected at the bottom of the skimmer at the center, from a pipe directed straight up (i.e. no tangential flow). This affords the same characteristics as a typical counter-current flow skimmer using an air stone at the bottom, except that the aspirator fills the column more thoroughly with bubbles. Water enters the skimmer at the top or side by gravity feed, or by a separate pump.
Variables that Affect Skimming Efficiency
There are many variables that can affect the operation of a protein skimmer including air flow rate, water flow rate, contact time, turbulence, temperature, pH, surface tension, specific gravity, column height and width, and mode of operation. The air injection method and bubble properties including size, distribution, and abundance are also factors. For more in-depth discussion of these and other factors that affect skimming, see Wheaton (1977).
Two major factors that aquarists can regulate are bubble size and contact time. Smaller bubbles provide more surface area per cubic cm for the surfactants, and rise through the water at a slower rate, providing more contact time. If the bubbles are too small, they no longer rise because they aren't buoyant, and they tend to dissolve in the water rather than collect surfactants (Moe, 1989).
As the surfactants coat the bubble surfaces, they effectively form a "skin" (Moe, 1989), that gives the bubbles stability of form, and maintains their size. This is caused by the effect surfactants have j on the surface tension of air bubbles. The more surfactants in the water, the smaller the bubbles will be (Spotte, 1979), until a supersaturation point is reached; then the surface tension that maintained the individual bubbles' size is broken, the bubbles group together, and become larger. If supersaturation occurs throughout the column, all the bubbles become large, the foam collapses, and efficient skimming ceases. Surface tension is also
broken by fats or oils introduced into the water, as from the aquarist's hand.
At the top of the foam riser tube, the heavy concentration of surfactants collecting there causes the bubbles to group together and become larger. As this top layer rises and collapses on itself, excess water drains back downwards, allowing the formation of a thick, stable, dry foam on top of a wetter foam layer. Wilkens (1973) discusses two basic types of foam that develop in effective skimmers. The first layer of wet foam is referred to as "standard" scum and the second as "protein" scum. It is the second layer, the protein scum, that concentrates harmful organic substances from solution. The standard scum is the layer of foam that develops as a result of protein molecules uniting with fine air bubbles. Protein scum is formed by organic substances, metals, etc. attaching themselves to the protein molecules in the standard scum. Protein scum is therefore found floating on top, where it gathers more organic compounds from the standard scum beneath it. Protein scum also leaves a solid deposit on the inside surface of the uppermost part of the skimmer column. The constant supply of air pushes the dry foam up and out of the contact column into a collection cup, or into a pipe that directs the foam to a waste vessel.
It is important that the air bubbles have as much contact time with the water in the column as possible, and contact time is enhanced by tall columns. We have seen skimmers for large systems that were over 2.6 m (8 ft.) tall! Of course this is not practical for most home aquaria, but the contact tube should be as long as possible. Long columns also allow more time for drainage to occur, producing dryer foam, especially with high volumes of air input (Wheaton, 1977). Skimmers placed in the aquarium or filter have limited height. This is why free-standing skimmers used outside of the aquarium are preferable. However, small protein skimmers with Venturi air injection can be very efficient because of the high density of uniformly sized small bubbles, and tangential injection of the air-water mixture into the contact column. Tangential injection of the water into the contact column can provide a swirling action that increases the contact time by slowing the upward migration of the air bubbles.
The swirling of water must not be too powerful at the top of the contact tube where the scum formation occurs. Turbulence there prevents proper separation of the dry foam. Lower in the column, high speed swirling of the air-water mixture causes a centrifuge
effect, which is employed to advantage in some small skimmer designs. These may be called "centrifugal skimmers'\ Centrifugal action causes a separation of the water and foam, the water migrating outwards because it is heavier, and the air bubbles collecting in the center of the contact column. A physical barrier installed to prevent the swirling action from extending to the top of the contact column, allows smooth uniform rising of the bubbles, and proper foam formation. Centrifugal action, which provides very good contact between the air and water in a relatively small space, has one minor drawback. Some refractory organic compounds (cellulose?) remain in the water, are cast outwards in the contact chamber, and coat the inner surface. The accumulation of this yellowish carbohydrate sludge must be cleaned periodically to maintain peak operation of the skimmer (see appendix B).
The pH of the water has an impact on foam formation. The pH of aquarium water is not constant, but follows a typical range over the course of a day. The operation of the skimmer, therefore, varies with the change in pH. The higher the pH, the greater the affinity of organic molecules to the surfaces of the bubbles because of enhanced electrostatic attraction. The structure of many proteins is also sensitive to pH, and extraction of a type of protein is greatest at its particular isoelectric pH (see glossary). Since proteins have different isoelectric pH values, some may be skimmed better than others in the aquarium's range of pH (Wheaton, 1977).
Specific gravity affects skimming because the dissolved salts and other compounds in the water increase the stability of the tiny bubbles by increasing the viscosity of the water. Specific gravity also affects electrical charge attractions of compounds to the bubbles, and the surface tension of the water. Surface tension increases with increasing specific gravity. Although skimmers aren't supposed to function wrell in freshwater, they can be made to work when there is sufficient organics (i.e. crud) in the water, that likewise enhances the stability of the tiny bubbles. Heavily stocked koi ponds, for instance, benefit from the application of protein skimming.
Temperature affects protein skimming as well. As temperature increases, surface tension decreases. At high temperatures the foam breaks more readily, so the formation of stable, dry foam is impeded. However, in the range of ideal temperature for reef aquariums, between 21-27 °C (70-80 °F), the temperature affects on protein skimming efficiency are subtle. Nevertheless, certain surface-active substances foam only within certain temperature
ranges. Therefore not all surface active materials will be removed from solution by the skimmer, and some will be removed more slowly than others because the temperature is not in the ideal range for them to foam (Wheaton, 1977).
The volume of air, water flow rate, and the design of the "neck1' (foam riser tube) and water exit pipe of the protein skimmer all affect its capacity to filter an aquarium. Increasing the volume of air increases the surfaces for attachment of organic compounds. This increases foam formation and the efficiency of stripping organics from a given volume of water. Increasing the flow of water through a protein skimmer increases the rate at which the aquarium water is being purified. Combining these two parameters, one can see that for a given size skimmer there is a maximum flow rate of water and maximum volume of air that can be injected to achieve the best skimming for its size and construction. The design of the neck and exit pipe of the skimmer are limiting aspects of its construction with respect to the ability to pump a high volume of air or water through. When high volumes of air are injected into a skimmer with a short, constricted neck, wet foam rapidly flows out the top and floods the floor. Likewise, increasing the volume of water flowing through the skimmer increases the waiter level in it, and thus pushes the foam level higher, with a similar flood-prone result. When the neck is not constricted, is veiy tall, or both, these increases in air and water flow do not cause a flood so easily, the skimmer can be packed full of tiny air bubbles, and it can process a good turnover of water. The size of the water exit pipe should be larger in diameter than the water input, so that increases in water flow readily drain out, and do not cause much increase in the water level in the skimmer (see figures 5.3 a&b). We have observed many protein skimmers operating well below the capacity possible for the size of the contact column. If the column is made of clear pipe, and you can see through the bubbles, then the skimmer is not being used efficiently. It should look like dense "whitewater". Higher air flow rates increase the available bubble surfaces until bubble coalescence occurs in the column at a rate faster than the generation of new surfaces (Wheaton, 1977). That limit of air input is never reached in typical protein skimmers because water and foam spew out the top long before. If increasing the volume of air slightly tends to cause the foam to flood out of the top of the neck, the design of the skimmer needs to be changed. Build a taller, wider neck to solve this problem, allowing the skimmer to be packed with high density of fine bubbles.
Size of the Skimmer
The size of the skimmer you construct depends on the size of tank you want to filter. A skimmer with a diameter of 15 cm (6 in.) and a height of 120 cm (4 ft.) can handle aquariums up to 600 litres (160 gal.) in capacity, using a flow rate of 1200 litres (315 gal.) per hour (Achterkamp, 1986). As a rule of thumb, we recommend that the entire volume of the aquarium be passed through the skimmer(s) at least once per hour, preferably more. Use this figure as a guide to determine the size and number of protein skimmers you will need. With an external protein skimmer, the size of the skimmer is not restricted by the size of the aquarium. This is a very important consideration because the larger the skimmer is, the greater the
Some people can never have enough skimming! A Sanders Helgoland skimmer at the Rotterdam Zoo, Netherlands. J.C. Delbeek.
Sanders Helgoland skimmer. J. Sprung.
volume of water that can be pumped through it. In addition, the greater the length of the skimmer, the greater the contact time between the water and the rising column of air bubbles, which is a limiting factor in the efficiency of any skimmer. The only restriction on the length of skimmers that use air stones is the strength of the air pump, since it has to pump the air down through that column of water. It is the height of the skimmer that is most important in determining the pressure required to mn an airstone; the diameter is not a factor (pressure = mass per unit area; P = m/a). You can put more than one wooden air diffuser in the skimmer provided you have enough room. Skimmers greater than 10 cm (6 in.) in diameter will work more efficiently with at least two blocks. Really giant commercial skimmers typically employ a type of Venturi or aspirator to supply the enormous amount of air required.
The diameter of the column does influence the operation of the skimmer. If air flow is constant, then increasing the skimmer diameter decreases the available bubble surface area per unit volume of water (i.e. if the pipe diameter is bigger, you have more water and the same amount of bubbles). If the air volume is also increased, then the skimmer with a wide diameter can process more water efficiently.
Purchasing the Skimmer
When purchasing a protein skimmer, there are a few points to look for. First of all, can the skimmer be easily and completely disassembled for cleaning? Can the skimmer cup and foam riser tube be easily removed for frequent emptying and cleaning? Check the length of the foam riser tube or "neck". If it is too short and narrow, the foam will not dry out and concentrate properly (see earlier discussion of the neck). Check the plumbing and make sure the inlet and outlet tubes are of sufficient dimensions to handle adequate flow. Finally, look at the types of valves included with the skimmer. Most brands available today come with simple ball valves. While these are fine for on/off operations, they are clumsy for fine adjustments of water flow. Much better are gate valves, which can be used to make very fine adjustments in flow rate. You will forever be fiddling with ball valves to maintain the proper foam height in your skimmer. Save yourself a lot of hair pulling: use gate valves. For designs that have a wide foam riser section (i.e. no restricted neck), and/or a large diameter water exit pipe, the type of valve is not so critical, since adjustments in water flow-do not have so dramatic an impact on the height of the water/foam level in the skimmer.
Installing the Skimmer
Because there are so many different types of protein skimmers, it follows that there are numerous ways to install them on an
aquarium. Protein skimmers will foam no matter where they are located on a system, but some methods do produce better results.
Protein skimmers designed to be mounted inside the tank can be very efficient, and eliminate the need for plumbing because the reef aquarium can be operated with no external sump. What could be simpler? The skimmer is mounted in a corner where it is least conspicuous. Larger aquariums using internal protein skimmers require multiple skimmer units, because the size of the skimmers limits their capacity. The main drawback to internal skimmers is aesthetics, though they can be hidden from view quite easily.
When multiple units are required, it may be preferable to locate the skimmers in a sump behind or below the aquarium, feci by surface skimmed water. Some internal skimmers are designed to draw water off the surface of the aquarium or sump where they are located, which enhances their efficiency.
External skimmers can be located far from the exhibit to avoid detracting from the appearance of the living-room decor, and they can be made really big, though efficiency is more important than monstrous size. The most ideal way to install an external protein skimmer is to feed it all of the surface skimmed water from the overflow, without any fomi of mechanical or chemical pre-filtration. Protein skimmers do collect and remove some particulate matter from the water. When mechanical filters trap particulate matter, the water circulates over it until it decays. Also, as mechanical filters j become coated with particulate matter they become better at trapping really fine matter, including carbohydrates and amino acids; so they trap and decay material that the skimmer could remove from the water. Chemical filters located before the skimmer would also trap and retain compounds that the skimmer could remove completely from the system. Chemical filter media (i.e. activated carbon) should be located after the skimmer. Since the skimmer is taking water from the overflow, be sure to install a strainer there to prevent fish from entering the skimmer after an accidental trip over the falls. A standpipe will retain some water in the overflow, so a fish that finds its way there can have room to swim around until you retrieve it. Of course the skimmer must have large enough diameter input and exit pipes, so that it can handle the water volume coming from the overflow.
When a skimmer is installed so that it receives water from the overflow drain, it cannot feed water directly back to the tank unless the return is fed by a pump or airlift. Generally the water from a protein skimmer installed this way drains to a sump. From there it is pumped back to the aquarium. The upper level of the exit pipe returning water from the skimmer is critical for the proper level of water inside the skimmer. If the pipe level is too low, the water level in the skimmer will be too low unless a valve on the exit pipe restricts the flow, causing the water level to rise. A design requiring such restriction of flow is a poor design because it limits the volume of water that the skimmer can process, and it is tricky to adjust. It is best to have a wide exit pipe with no restriction, with a fixed water level that is elevated only by the volume of air pumped into the column.
External protein skimmers can also be installed in such a way that they receive water directly from the pump that sends water back to the aquarium from the sump, with a gate valve on the feed to regulate the volume the skimmer receives. Be aware that when the power goes off, some water will drain back to the sump from the skimmer. Be sure that the sump is of sufficient size to handle this back flow. The water exiting the skimmer may be directed to the aquarium or back to the sump, depending on the location of the skimmer and its height.
Alternately, a dedicated pump can be used that draws water from the overflow chamber, sending it to the skimmer and back to the aquarium. In order for this design to work, a second hole or a siphon can be installed in the overflow chamber to feed the dedicated pump. The main drain of the overflowr is separate, and simply drains wrater to the sump. A standpipe on the main drain maintains a static water level in the overflow chamber. With this design it is possible to achieve two advantages. One is that the dedicated pump can be of greater capacity than the pump that returns water from the sump, allowing the tank volume to be processed more quickly through the skimmer. The second advantage is that the surface skimming action will be enhanced by the increased volume of water flowing over the overflow. This will also raise the water level in the aquarium slightly. Don't forget to have a strainer on the suction for this pump to prevent mincing fish or invertebrates that wander over the overflow.
The Skimmer in Operation
Once you have the skimmer connected in-line, if it is an air-driven model, connect the air supply to the air blocks and aim on the air pump first before letting water into the skimmer. This prevents the wooden air block from being presoaked; it's much harder to start a wet wooden block under 120 cm (4 ft.) of water than a dry block. You will have to watch your skimmer closely at first to make sure
that the flow is just right. Too great a flow will result in a full foam tube and collection cup, a wet floor and an empty aquarium! To guard against a mess on the floor, drill a hole near the top edge of the collection cup, and install a drainage aibe of at least 1/2 in. diameter that leads to a large waste container. Do not allow the skimmed off material to flow back into the aquarium; it is toxic when so concentrated.
Figure 5.3a External Air-driven
Skimmer Installation
After Nilseri 1993
|
L |
„ r^ |
|
E |
- A
AQUARIUM
A. Water Inlet, 20mm dia.
B. Water Outlet, 50mm dia.
C. Main Tube, 110mm dia. x 1500mm length
D. Screw Coupling with O-Ring
E. Rising Tube 110mm dia. x 400mm length
F. Scum-cup with drain to sink
G. Wooden Airstone(s)
H. Air Inlet
Figure 5.3b External Venturi Skimmer Installation
Pump with Venturi
Water \
Water \
Cioseup View of Venturi
AQUARIUM
Pump
A. Water Inlet
B. Water Outlet
C. Outlet of Water & Air Mixture
D. Screw Coupling with O-Ring
E. Rising Tube
F. Scum-cup with drain to sink
G. Air Inlet to Venturi
H. Inlet for Venturi Pump
You should notice a dark brown fluid and sludge accumulating in the collection cup after only a few days. Initially you might have to empty the collection cup 2 times a day, but the output will soon slow down appreciably. The skimmer is still working, there just isn't as much left to remove, that's all. You'll notice that the skimmer will begin to work furiously after feedings, water changes, addition of trace elements, or the addition of fresh live rock (boy will it ever foam after you put in live rock!). A reduction in output from the skimmer is not always caused by the lack of substances to remove from the water. In time the skimmer becomes dirty, and this impedes its performance.
In order for your skimmer to keep working at peak efficiency it must be kept clean. Every few days or about once a week, depending on how quickly scum accumulates, the foam collection tube should be scrubbed clean. Wooden air blocks should be examined and replaced if necessary, about once per month. Every 3 to 4 months the main body tube may require some scrubbing. A bottle brush works fine for the main and foam collection tubes while a coffee percolator brush works great on smaller tubes. The inside of the skimmer must be kept clear of algae and any build-up of organic material on the sides. For transparent skimmers, algae growth can be impeded by covering the main body with some opaque material. For additional information on protein skimmer maintenance, see appendix B.
There are a few things to keep an eye on when using a protein skimmer. First of all. the continuous removal of small amounts of
sea water by the skimmer, along with replenishment of evaporated water with freshwater, can lead to a gradual lowering of salinity. Therefore, the periodic addition of sea water to the make-up reservoir may be necessary to maintain the desired level of salinity.
Secondly, efficient skimmers can remove some trace elements as j we mentioned already. The regular addition of trace elements may be especially necessary when protein skimming is used (see chapter 8). Finally, the addition of certain buffers, vitamins and molecular adsorption filter pads can cause the skimmer to foam excessively. Rinse prefilter material and molecular absorption pads in pure freshwater before using them, and add buffers or vitamins very slowly, in small amounts.
At the introduction of the topic of protein skimming, we described the natural process of wind and waves generating foam that washes up on shorelines. The foam that collects around coral
reefs, best observed at low tide on exposed reef flats, plays a role in nutrient exchange of carbon, nitrogen, and phosphorous. Some
Of the foam may be broken down by intense UV wavelengths at the water surface that are capable of breaking chemical bonds (much as ozone does), but most of the foam is washed by tides and wind to inner areas of the reef and associated seagrass and mangrove ecosystems. Proteins, carbohydrates, trace elements, bacteria, and other "stuff attached to bubbles may be a significant source of food to filter feeding or particulate feeding organisms in these areas, and they may use the nutrient and element concentrating feature of bubbles to advantage. Gorgonians, for example, do capture and ingest tiny bubbles from the water (J. Sprung, pers. obs.). They may derive significant nutrition from the substances attached to the bubbles, and they can easily expel the excess gas collected. It is not known whether other filter feeders such as clams or tunicates utilize tiny bubbles this way. By demonstrating its obvious occurrence in the natural setting, we wish to emphasize that protein skimming is a natural process, and that it is easily duplicated in the care of aquarium systems.
We are satisfied that protein skimming is the simplest and most efficient means of water purification for the maintenance of live corals and for the creation of model ecosystems. So much benefit to the aquarium is provided by a device that simply mixes the water with fine bubbles of air.
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