Everything You Ever Wanted to Know About Protein Skimming
By Jason Kim
Protein skimming has rapidly become one of the hottest topics in the marine aquarium industry. And for good reason, since this method of “filtration” is one of the easiest, sure-fire ways to promote the health of a saltwater system. Although most would agree that a protein skimmer, or foam fractioner, is a must-have on any marine tank, there nevertheless remains a healthy debate over the subtle intricacies of just how to accomplish the process of skimming. This debate has led to a small amount of confusion among some beginning aquarists, and even experienced veterans in a few cases.
Protein skimming is nothing new – advanced aquarists and professionals have been using the technique for decades. Other fields unrelated to the aquarium industry have also used forms of foam fractionation to separate compounds from liquids. Only in the last few years, however, has the widescale market for skimmers exploded. New models and methods seem to pop up with the frequency of political scandals these days. This has led to a great deal of controversy as to who makes the best skimmer. This is not an easy question to answer. An aquarist looking for a small unit designed to skim a 15 gallon tank would not agree that a six foot tall venturi model would be the best choice. On the other hand, a hobbyist with a 100 gallon reef would hotly argue that this very skimmer is the best choice! Who is right??
They both are… There is no such thing as the “best” skimmer!
In many ways, protein skimmers are a lot like automobiles. Some people lust after a two door sportscar, while others might desire a four-wheel drive truck. Although neither automobile is quantitatively better than the other, each fills the needs of a different type of driver best. Although you might not realize it, the skimmers on the market today vary as much as a Toyota Truck and Porsche Boxter do. And, if we carry this car analogy just a bit farther, there are the Mercedes Benzes of the skimmer world, just as there are the Ford Pintos (our apologies to the Ford Motor Company).
The first thing to do, then, is to examine the criteria which are important to you. In most cases, the two most important factors that will be considered are cost and size. Money doesn’t grow on trees, and most people don’t want a skimmer which resembles a Giant Redwood poking out of their living room floor.
The cost issue is a no-brainer – all else being equal, purchase the most inexpensive skimmer (unless you’re Bill Gates, that is…). Another factor to consider are the ongoing costs. Protein skimmers require pumps, which suck up electricity. Some of the high performance models sold today use hefty, energy-sucking water pumps, so be sure to calculate this cost before your purchase. Another ongoing cost might be maintenance related. Some types of skimmers will require regular, scheduled cleaning and/or replacement of air stones. There is more on this below…
Choosing the right size skimmer is a bit more complicated, as several factors need to be considered. First of all, do you have a sump or will the unit hang off the back of the tank? What size skimmer footprint (including pump) will your sump accommodate? How tall of a skimmer will fit under your cabinet? Can the skimmer be placed outside of the sump, and if so, is any special plumbing required? Before purchasing a skimmer, carefully consider these questions and make the necessary measurements/calculations to help you choose the best model. Although size doesn’t always correlate with performance, a general rule of thumb says that a larger skimmer will usually outperform a smaller one (Don’t worry guys, we’re talking skimmers here…!).
Another note about your budget… A lot of new aquarists will invest £250 on “entry-level” equipment and then immediately spend £150 on livestock, all within the first month or two. This has got to be the biggest mistake that nearly all inexperienced hobbyists make, and it usually ends up costing truckloads of money in the long run. After working for several years in a tropical fish store, I can’t tell you how many people ended up pawning off their original, shoddy equipment and investing in quality products. That £50 queen angel you have your eye on might look great, but if you skimp on your filtration set-up you are effectively playing Pomacanthus Roulette.
So let’s say you set your budget at £120 for a skimmer which must be no taller than twenty inches, the size of your aquarium stand. Remember, most units require occasional servicing – don’t buy a skimmer that will fill the sump like a fitted glove. Now we can talk about the real nitty gritty…the most complex factor you will be forced to consider when purchasing a skimmer – THE METHOD. This, unfortunately, is where it gets tricky!
First of all, no matter where you go, who you talk to, or what you read, you’re going to hear different and possibly contradictory reasons why Brand X is better than Brand Y, or why Method A is better than Method B. The fact is, there simply is no such thing as a “best” skimmer. If your budget is £50, ninety percent of the skimmers on the market aren’t even an option! Since nearly every skimmer is best given a different set of circumstances, there is really no reason to enter a silly debate over brand names. But we can talk about the meat and potatoes – what should you look for in a protein skimmer.
Protein skimmers benefit water quality by facilitating two major processes – gas exchange and export of organic wastes.
These are both critical to the maintenance of optimum water conditions. The sad truth is that no captive system can ever truly duplicate the conditions found on a natural coral reef. Effective protein skimming can, however, bring us very close to this desired goal. The beneficial gas exchange that goes on inside of a protein skimmer is largely dissolution of atmospheric oxygen into the aquarium water. Other gases like nitrogen and hydrogen also play roles here, but we aren’t as interested in them. Some evidence even suggests that ammonia might be “blown-off” into the atmosphere during protein skimming. It is important to keep dissolved oxygen levels as high as possible, since most organisms (fish, corals, inverts, etc.) we keep are used to these conditions found in the wild. Waste removal is accomplished through chemical interactions between charged particles of air and water, which allows proteins, trace elements, and other compounds to become attached to air droplets. These protein-laden air droplets then coalesce into a thick foam which is skimmed off into a collection cup.
Although it is certainly important to understand how and why protein skimming works, the actual physics and chemistry of the process involve some pretty scary equations. Many of the factors involving effective protein skimming are far from being understood, even to this day. Luckily, however, the basics are relatively easy to understand…
In short, the four main factors which govern a protein skimmer’s effectiveness are…
Overall flow-thru rate
Quality of contact time
Pretty simple, right? Well, it gets more complicated…
First let’s talk about bubble size. Smaller bubbles are preferred over larger ones since they offer a larger surface area to volume ratio. Since the nuts and bolts of protein skimming takes place on the surfaces of air bubbles, our goal is to maximize this variable by introducing the smallest bubbles possible. Of course, at some point the bubbles could become so tiny that they would actually stay dissolved in the water and cease to produce foam. For most purposes, it is nearly impossible to generate bubbles this small. The same logic follows for bubble quantity – since larger numbers of bubbles mean increased surface area, we want to inject as many bubbles, overall, as physically possible. Once again, it would be possible to inject too many bubbles (imagine a protein skimmer packed so tightly with bubbles that it was almost “dry”. Given the technology available, however, this has yet to become a realistic problem.
The overall flow-thru rate and quality of contact time are not as clear cut as the previous two measures. Some have reported that high flow-thru rates (several hundred gallons per hour) yield the best results. Others argue that a slow, leisurely flow-thru rate facilitates the best skimming. Most likely, both schools of thought are correct in their own way. The actual chemistry behind the mechanism of protein skimming tells us that, in order to remove the maximum amount of waste possible per bubble, we want a very, very slow flow-thru rate. This makes perfect sense… If the flow-thru rate is slow, each individual air bubble can react with the water for a longer period of time. Air bubbles, after all, don’t become saturated with proteins immediately. Research suggests, in fact, that they continue to become coated with organic compounds for up to 12 minutes (Harker).
The problem, and this is why the “high flow-thru rate” school of thought is also partially correct, is that aquariums are closed systems which need to be “cleaned” at an efficient rate of speed. Imagine holding a vacuum cleaner over the same section of carpet for ten straight years. Sure, that little square of shag might be extremely clean once you are done, but what about the rest of the house?? Our goal is not to maximize the amount of protein removed per bubble, but to maximize the amount of protein removed per unit time. An efficient protein skimmer, then, is one which removes the largest amount of waste in the shortest time possible. High flow thru-rates are beneficial because they allow large amounts of the aquarium water to be processed rapidly. Although less protein is removed per bubble (because the air and water are not being allowed to react for very long), it is possible to treat the entire tank volume in a short amount of time. There is probably a middle ground which yields the best results. It is important to note that both high and low flow-thru rates can and have been used effectively, and neither method is definitely better than the other. Our own research with different flow rates and air-injection methods suggest that for larger tanks (100+ gallons), higher flow-thru rates are better suited than slower ones. This might be due to the added benefit which high flow-thru rates provide; more thoroughly oxygenated water. The real answer is far from clear, however.
By now you should have a general idea about the main factors which make some skimmers more effective than others. There still remains one very large problem, however, and that is the method. Open the latest issue of FAMA and you’re bound to see ads for over ten different skimmer brands. Do they look the same? Heck no! Do they work the same? For the most part, yes. Every skimmer has its pros and cons, however.
Our market research suggests that there are six main factors which are important to customers. These are, in no specific order…
Ease of Use
There are basically five different types of protein skimmer. These are:
The Air-Driven Skimmer…
This method of skimming (sometimes referred to as the counter-current method) is probably the simplest and has been used for decades. A standard air pump pushes air through a porous block (either made of wood, glass, or ceramic material) which produces fine bubbles. Usually, a separate water pump is required to provide flow through the skimmer and back to the aquarium or sump. The main advantages of the air-driven skimmer are its low initial cost and quality of contact time. Wooden airstones produce tiny, wispy air bubbles that provide a high air-surface to water ratio. Since flow-thru rates are kept relatively low in this type of skimmer, bubbles are allowed to react with water for long periods of time. This is the hallmark of an air-driven skimmer, which feature a high “protein removed per bubble” ratio. Furthermore, since control of the water flow-thru and air-flow rate are often independent of each other, they can be manipulated individually to yield the most efficient skimming possible. Another positive feature of air-driven skimmers is that the air bubbles are typically allowed to react with the water for very long amounts of time.
Unfortunately, all skimmers have their drawbacks. Air-driven skimmers are utilized most efficiently when the flow-thru rate is low – which insures that the contact time between air bubbles and water is maximized. Although this yields a high “protein removed per bubble” ratio, the “protein removed per unit time” ratio can actually suffer as a result. This might be why larger tanks seem to do better with the higher flow design skimmers. Furthermore, most air pumps available to hobbyists are simply not powerful enough to deliver tremendous quantities of air to the skimmer. Air-driven protein skimmers can certainly be utilized efficiently and will produce good results, as long as we keep these factors in mind. Air-driven skimmers also require regular maintenance – wood air stones must be replaced relatively often in order to keep the skimmer working efficiently.
The Venturi Skimmer
The “venturi skimmer” is actually a general term which covers a broad class of different designs. Most skimmers on the market today utilize the venturi effect in some form or another. So what is a venturi, anyway?? It’s just a high-tech word for a special-shaped tube which draws in air. A good venturi can introduce fairly large amounts of air. Venturi skimmers also feature higher flow-thru rates, and require less regular maintenance, both advantages over the air-driven design. These skimmers are not without their drawbacks, however. They can be loud (imagine the sound of air being blown through a small straw), and the venturi opening can clog in a relatively short amount of time, which adversely affects performance. Most high performance venturi skimmers are either very expensive or very large, making space and budget considerations a factor.
The Needle-wheel skimmer
This type of skimmer is actually a hybrid, since it uses a venturi to initially introduce air into the water. Once air is injected into the skimmer, it is drawn through a water pump with a special impeller designed to chop incoming bubbles into a finer mist. These needle-wheel impellers can really hack bubbles apart, which is great since small bubbles are ideal for efficient skimming. The better needle-wheel skimmers on the market are notorious for building a dense, milky froth. This makes for efficient skimming in a relatively compact package. As a result, this type of skimmer is extremely popular, with new designs popping up every month or two. Although the air and water flow rates in these skimmers are high, they still fall short of the air-induction style skimmer which we will talk about next.
The Downdraft skimmer
Initially, several different companies manufactured similar versions of the now popular downdraft-style protein skimmer. Since A.E. Tech owns the patent rights on the design, most of these companies are no longer competive. There remains one brand which utilizes a hybrid variation of the venturi and downdraft concepts which also enjoys quite a bit of success, and does not infringe on A.E. Tech’s patent. Downdraft skimmers feature high flow-thru rates. They also inject large amounts of air into the water. A typical downdraft skimmer can cycle the entire volume of a 100 gallon system several times an hour. This makes for excellent oxygenation and ensures that all of the water is treated in a short amount of time.
Downdraft skimmers are often advertised as being the “best” skimmer on the market. This, in on our opinion, might be a bit of an overstatement. Although this design works well, it is not without its disadvantages. These skimmers are notorious for being large… many models stand several feet tall. They also require a very strong pump to run correctly, in many cases a pump which is expensive and energy-thirsty.
The Spray Induction skimmer
Remember the six main factors to consider when buying a skimmer? These were performance, initial cost, ongoing cost, maintenance requirements, size, and ease of use. After testing nearly every other type of skimmer on the market, we have found that the Spray Induction design yields excellent results when all six of these factors are considered. This design seems to inject the largest amount of air for a given pump size, and it is extremely short and compact. There are virtually no maintenance requirements or parts to replace. Once broken in, this type of skimmer is truly a “set and forget” piece of equipment.