2.0 Filtration and Equipment

2.1 Live Rock

Live rock is simply old reef substrate that has become the home to multiple small plants and animals. Pieces vary in size and shape from baseball size to dinner plate size in typical tanks. In large tanks (> 500 gallons) very large pieces of live rock tend to be used.  These pieces may individually weight up to 85lbs (about the limit of what one person can handle).

The use of live rock greatly increases the bio-diversity in a tank.  However, its primary purpose is to provide a home for bacteria that provide the biological filtration for the aquarium.  Cheap rock has low amounts of coralline algae and tends to grow hair algae well. It may be suitable for a soft coral only tank.  Hair algae free coralline encrusted live rock (high quality Florida and/or pacific (Marshall and Tonga Island) rock is highly desirable. "Berlin" style tanks use high quality live rock (and protein skimming) as the primary filtration method with great success.

Although an old rule of thumb states that 1-2 lbs of live rock is required per gallon of tank size, the wide range of available rock makes the rule pretty inaccurate. It is suggested that a visual method be used, consuming approximately 1/3rd of the tank volume with rock - leaving 2/3rd of the volume in open water. You should probably only use the rule of thumb as a sanity check. For instance, 10 lbs of the best rock would be too little for a 75 gallon tank, no matter how good the rock is. Likewise, 300 lbs would be overkill.

Live rock is typically "cured" prior to introducing other life forms in a tank. This curing process is, in its shortest form, simply a period of time to allow dead and dying organisms on the rock a  chance to decay. Any time live rock is moved, some organisms will probably die. Shipping rock submerged in oxygenated water (VERY expensive) is the only practical way to minimize this die-off.

Live rock should be cured in a container with excellent protein skimming, activated carbon, EXCELLENT oxygenation and water motion. There is a very real danger of anoxia when freshly shipped live rock is placed into a curing vessel. Unless the dissolved oxygen concentration is kept high in the curing vessel, the bacterial bloom fed by the initial die-off will cause the curing vessel to become anoxic, and even more life will perish. Therefore gas exchange and water motion are crucial. Protein skimming helps to remove organics before they are consumed by bacteria.  Addition of a cycled biological filter may reduce the severity and lethality of the ammonia spike when curing live rock.

It is recommended that fresh live rock be allowed to cure for at LEAST one month prior to the introduction of any other life forms in a newly setup aquarium. There may well be advantages to waiting between three months and a year, with the tank running in normal mode (full circulation, heating, lighting, etc), before adding other life forms in order to allow the biodiversity naturally present on the rock to stabilize.
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2.2 Protein Skimmers

Protein skimmers are devices that mix large volumes of air and tank water to produce foam. This foam is then collected and disposed of. The foam will contain a fair amount of particulate material, lots of organic material that would otherwise breakdown and pollute the tank, and unfortunately some trace elements like iodine. Besides removing organic material, skimmers play a key role in maintaining proper O2 and CO2 levels in tank water. For instance, having a large skimmer that processes a large amount of air will allow larger quantities of kalkwasser addition, both due to increased evaporation, and due to improved CO2 absorbtion capacity.

Required equipment. Don't undersize. Common wisdom is that you can't overskim a tank. Recent developments in using down-draft style skimmers, with ETS being the first commercial instance, have raised the possibility that it's now possible to overskim a tank.  This is stated with a lot of caution, we still feel that its impossible to overskim using airstone or venturi driven skimmers of reasonable size. (Using a 8" x 6' counter-current skimmer processing 600 gph of air on a 20 gallon tank could overskim it - be reasonable!)

Unfortunately, there is no formula to determine the required size of a skimmer. Amount of organic waste generating organisms (fish, coral, live rock, etc.) will obviously be the primary variable.  All skimmers should be filled with TINY bubbles and have a milky white appearance. Any skimmer that doesn't match that requirement is not working optimally.

There are some basic rules-of-thumb on minimum skimmer sizing.  However: A skimmer should process at least one tankful of air and one tankful of water per hour. For most tanks, the water rate is easy. The air rate however is not. Most counter current (explained below) skimmers are under-supplied with air. If you have a sealed skimmer where the air can only exit from one fitting, its easy to measure the flow rate. Simply take a large plastic bag (something in the 2 gallon size works well), empty it, and place it over the air exhaust port. Time how long it takes to fill and do the math.  Three basic styles of skimmers exist: counter current air driven, venturi driven, and down-draft. All styles work fine, all have tradeoffs. All require some tuning. Expect to spend some time over the first month or so learning how to keep your skimmer tuned.
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2.2.1 Counter Current Air Driven Protein Skimmer

These skimmers usually require three pieces of equipment typically not sold with them: an air pump, air stones and a water pump.  Total skimmer cost depends upon the kinds of equipment needed to run the skimmer properly.

The water pump injects the water to be skimmed into the unit.  Some people use gravity to feed surface overflow water to the skimmer or divert part of the main circulation pump's return flow into the skimmer to eliminate the need for a dedicated pump.  Otherwise a powerhead in the sump usually suffices for the water pump. Some arrangement should be made to gather surface water for all forms of skimmers.

The air pump must be large enough and a sufficient number of air stones must be driven to make the skimming column milky white. In some smaller skimmers one medium sized air pump like a Tetra Luft G and one air stone will be sufficient. Other skimmers need a lot more to perform optimally.

Speaking of air-pumps, we find it baffling that folks who would consider spending $250 for a water pump to drive a different style skimmer totally reject spending anything on that order for an air-pump.  When comparing skimmers, please be reasonable. Don't expect a counter-current airstone skimmer driven by a Tetra Luft to function as well as a venturi or down-draft skimmer driven by an Iwaki RLT-75 water pump costing around 10 times as much.

Air driven skimmers should use limewood air stones which will need to be replaced from time to time. Cheap limewood air stones have a reputation of needing to be replaced much more often than high quality stones. Coralife limewood air stones have a good reputation. Air stone replacement rate depends on your tank and skimmer; some people need to change them every 2 weeks others only after 3-4 months. It is believed that having a high air-flow prolongs the life of airstones. Some folks are experimenting with using VERY high quality fine-pore ceramic airstones, such as those available from Aquatic Eco-Systems.

A.J. Nilsen recommends a 1x tank volume per hour turnover of both water and air by counter current air driven skimmers. Others feel each skimmer has an optimal rate of air and water processing and that if more skimming is desired then more or bigger skimmers should be added rather than trying to operate the current one beyond its optimal performance range.  Some hold that any skimmer under 4' high and 4" in diameter is too small for anything over about a 20 gallon reef.
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2.2.2 Venturi Protein Skimmers

These skimmers use the Bernoulli effect of the venturi valve to inject air bubbles into the water. This obviates the need of an air pump and air stones. The penalty is that a relatively large, high pressure (read expensive and power hungry) dedicated water pump is mandatory for the venturi unit to inject sufficient amounts of air.

A particular commercial venturi skimmer may or may not come with a water pump. If it does supply a pump, it may or may not be sufficiently large to run the skimmer properly. At least some of the venturi skimmers easily available are not very well designed.

Venturi valves require occasional cleaning of the air opening.  This is as simple as reaming the opening out with a pipe cleaner every few days. An acid bath may be required if the unit clogs or gets coated with mineral deposits.  Most venturi style skimmers are more compact than CC skimmers.  Manufacturers state that they are more efficient, since they (supposedly) inject more air. Many suspect that design constraints (back pressure severely affects venturi performance) have more to do with the manufactured height (who would want a top injected 4' skimmer with air only in the top foot of water?). Properly designed venturi skimmers are tall to maximize air contact time, and require pumps that can handle backpressure.
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2.2.3 Down-draft Skimmers

This style skimmer was first commercially introduced as the ETS skimmer.   Currently there are several on the market. This skimmer using a high pressure stream of water injected downward into a column of bio-balls to suck air into the water stream and break the air up into tiny bubbles. They do this VERY well, with some casual testing indicating air movement on the order of 10 times the volume of equivalently pumped venturi models.

The cost of ETS skimmers is relatively high, but expected to drop as competitors enter the market. Cost of operation is high due to the need for a huge water pump. The use of Iwaki 55s and 70s on base unit ETS skimmers is common. This size skimmer is appropriate for a 70-135 gallon tank. e.g. The skimmer pump may be larger than many use for the main tank circulation.

Construction of down-draft skimmers is easy, designing one to function optimally is not.

As previously mentioned, this is the first style skimmer where the authors of this FAQ have even considered the possibility of over-skimming a tank. With this style skimmer, bigger may actually not be better...
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2.2.4 Protein Skimmer Considerations

Below are some pros and cons of three styles of skimmers. Some people will debate some of the statements. 

Venturi skimmers, due to the large water pump needed, have a higher initial purchase price than CC units for the same amount of skimming. Many venturi skimmers are poorly designed, with woefully inadequate pumps to drive the venturi valve. Remember: Its not the technology that makes a skimmer good, its the amount of air and water processed that makes it good. The technology is just a method to reach that goal. There are plenty of counter current skimmers that out-perform venturi based skimmers, and visa-versa.

The operational cost of a venturi unit is basically just the electricity bill. A CC unit must sum in electricity consumption for the water pump and air pump (usually small) plus air stone and diaphragm replacement. Which one is more cost effective for you depends upon which equipment you had to buy to run the skimmer properly, your electricity rate and how often air stones need to be replaced. Most people find CC skimmers less expensive to both purchase and operate for the same amount of skimming.  Venturi skimmers are less cumbersome in appearance and in operation. They are usually smaller and quieter. They are on the whole more hassle free. The powerful pump required for venturi skimmers may, however, add considerable heat to the water.

When large down-draft skimmers are driven by an appropriate pump, they outperform venturi skimmers that use pumps of similar power consumption. That is, they are more "efficient". Once setup and past their 3-14 day break-in period, down-draft skimmers require no maintanence beyond the periodic cleaning that all skimmers require.   There is some debate over down-draft skimmers vs properly run counter-current airstone skimmers. A counter current skimmer can be provided with a similar air and water flow for somewhat less initial money. Note that we are NOT comparing $1200 ETS systems to $30 Coralife co-current skimmers here, but rather systems about the same physical size.

Of the three skimmer designs, counter current skimmers are the most plankton friendly. Although some small plankton will be removed as particulate material, it is at least conceivable for plankton to survive the trip through the water pump and through the bubble column. Venturi skimmers are much harder on plankton, since significant pressure is applied to them as they pass through the venturi valve.

Down draft skimmers are assumed to kill anything larger than single cell organisms that pass through them due to the force and mechanical stress the water is exposed to.

One general note on water pumps: The amount of heat added to the water varies by brand, design, usage, and placement. Basically, the more efficient the pump (gallons delivered at a given pressure for a given power usage), the cooler it will run. Restricting the output of the pump will generally increase the water temperature.  (Never restrict the intake of a centrifugal pump!) Obviously, an air cooled pump will increase your tank temperature less than a submersible (and therefore tank water cooled) pump will.
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2.3 Granular Activated Carbon (GAC)

There is some debate about its usage. Most use it at least a few days a month, some continuously. Many brands have problems with phosphate leaching.  GAC has the ability to very rapidly remove dissolved organic compounds which cause the water to yellow. Indeed, failure to remove these compounds is a excellent way to determine when your carbon has been exhausted. A simple test consist of collecting (even temporarily) 5 gallons or so of tank water in a white plastic container. If the water appears yellowish, the carbon should be replaced.

WARNING: If the tanks water is significantly yellow, carbon should be replaced VERY slowly, like a gram-per-gallon at a time. Failure to do this may drastically improve the water's clarity, allowing more UV light to reach the organisms. Corals have been known to bleach and die after large carbon changes due to this rapid light transmission change.
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2.4 Other Chemical Filter Media

X-Nitrate, X-Phosphate, Polyfilters, Chemi-pure, etc. - probably are not needed in an established, balanced reef aquaria. A prominent manufacturer of these materials was either unwilling or unable to supply capacities for removing the named compounds from seawater. They may cause adverse reactions in some inverts.
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2.5 Mechanical filtration

This is an area of interest currently being debated. Originally the FAQ stated:

Good idea to pre-filter skimmer water. Floss works fine and is cheap and disposable. Sponges work well, but require cleaning twice a week or so. Natural sponges with a medium fine or fine pore size are recommended. Some people don't use mechanical filtration, allowing detritus to settle in places for removal by siphoning. Some of these people make dedicated "settling tanks" to trap debris in a convenient place.  Julian Sprung suggests not pre-filtering skimmer water as skimmers will remove particulates (rather than trapping them as a pre-filter would do). Spotte confirms this and terms this filtering mechanism as 'froth flotation'.

Many members of the group of authors do not use mechanical filtration.  They believe that such systems filter out the plankton that is used as food by many marine organisms. Some members use "live sand" setups, with detritivores. Others routinely siphon accumulated detritus.

Use of a mechanical filter for short periods may help when attempting to resolve specific problems, such as a hair algae outbreak.
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2.6 Under Gravel Filters (UGF)

Not appropriate for a Reef Tank. Although they will work for 6 months or so, eventually detritus buildup will cause a nitrate problem. Long term, it's virtually impossible to keep nitrates below about 40 ppm NO3- which is way too high for corals.

Remember, NO3- is an indicator for other waste compounds (e.g. dissolved organics) which are not easily measureable and these compounds will also be present in abundance.
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2.7 Reverse Flow UGFs

An attempt to solve the detritus buildup problem associated with normal flow UGFs. It's a good idea that doesn't work well in practice. This system has problems with uneven water flow due to channeling within the bottom gravel.
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2.8 Trickle Filters

Also known as Wet/Dry Filters. An improvement over UGF and RUGF filters. Nitrates can be kept low (say, around 5 ppm) with adequate water changes. It does not seem to be possible to keep nitrates very low (less than 1 ppm) if a trickle filter is the sole biological filtration. Those that report less than 1 ppm normally have adequate live rock, and find that their nitrates remain low even (and often get lower) when they remove all the bio-material from their trickle filters (turning them into plain sumps, useful for holding carbon and as a water reservoir).
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2.9 Algae Scrubbers (long)

Summary: The jury is still out, even after several years of  experience. Some species of corals seem to do fine, and occassionaly even better, in algae scrubed tanks. Other species simply die.  In any case, we still recommend this only for experienced aquarists who fully understand the risk they are taking. What's included below is a decent summary of how folks felt a couple of years ago. Most have softened their view abit since then. Few consider the systems the ultimate solution...

In most healthy natural communities, particularly coral reefs, dissolved nutrients are scarce. In aquaria, by contrast, nutrients in the form of dissolved inorganic nitrogen, or DIN, (a collective term for ammonia, nitrites, and nitrates) accumulate very rapidly as fish and other organisms excrete these wastes.  The most basic problem in any aquarium is limiting the accumulation of DIN.

In reef aquaria, DIN is consumed by the community of organisms on the live rock. It is uncertain what relative contribution is made by bacteria as opposed to algae, but it is certain that the live rock community as a whole can remove a substantial amount of DIN from a reef aquarium. In fact, it is quite possible to run a reef tank with no biological filtration (DIN consumption) other than that which takes place on the rock. This method is part of what is now known in the United States as the "Berlin school" of reefkeeping.

Other schools of thought utilize additional biological filtration in separate filters. Traditional reef tanks supplement the filtration provided by the reef (often not acknowledging the role of the reef itself) with bacteria-based trickle filters. Many readers probably learned this technique first, as it has been the dominant method in the United States amateur hobby for some time.

Yet another approach uses algae, which are also capable of utilizing inorganic nitrogen directly. An algae filter, or algal scrubber as it is usually called, is simply a biological filter which utilizes a colony of algae rather than bacteria as consumers of inorganic nitrogen.

Algal scrubbers are not new; they are discussed in Martin Moe's (1989) excellent Marine Aquarium Reference: Systems and Invertebrates, for example. However, algae filters have been regarded in the past as too bulky and inefficient to be the sole filter for a aquarium. The recent surge of interest in algal scrubbers seems to have been generated by Adey and Loveland's book Dynamic Aquaria (1991). They discuss both techniques which allow an algal scrubber to be compact and efficient and also a number of arguments as to why they are preferable to other filtration methods.

One reason to use an algal scrubber according to Adey and Loveland is that it mirrors the way DIN is cycled in nature. They claim that perhaps 70-90% of the DIN in reef communities is consumed by algae, rather than by bacteria. The two methods produce rather different water chemistry; for example, algae are net producers of oxygen and remove carbon dioxide, while a bacterial filter consumes oxygen and produces carbon dioxide. They argue that it should be easier to maintain the type of water chemistry found over a natural reef by relying on an algal scrubber.

Also, algae remove the nitrogen from the water in order to build tissue, while filter bacteria simply put it into a less toxic form. The excess nitrogen can be removed completely by periodic algae harvests, while dissolved nitrogen in the form of nitrate is not as easy to remove. Adey and Loveland claim that their methods can bring levels of DIN down to a few hundredths of a ppm, far below (in their opinion) the levels reachable with other methods.

A related argument in favor of algal scrubbers is that stability in natural ecosystems comes from locking up nutrients in biomass, not in allowing it to be free in the environment. An algal scrubber does precisely this, while a bacterial filter converts it to free nitrate dissolved in the water.

A final reason to use an algal scrubber according to Adey and Loveland is that many other kinds of filtration (including protein skimmers) remove plankton from the water. An algal filter naturally does not do this, and can actually provide a refuge for some forms of plankton. The importance of this effect is, however, a matter of some debate.

As compelling as some find the above arguments in theory, there seem to be serious problems with algal scrubbing in practice.  Many attempts by public aquaria at implementing reef tanks using only algal scrubbing have been failures. In particular, it seems difficult to find successful long term success with Scleractinia (stony corals) in such tanks, and those success stories which can be found are quite difficult to verify and often contradicted by others.

Various public and private aquaria have used algae scrubber filters on their reef aquaria, with disastrous results. The microcosm at the Smithsonain Institution has yet to keep scleractinia alive for more than a year. While Dr. Adey has stated how well corals grow in this system, those viewing the system have failed to find these corals. In an interview with Jill Johnson, one of the techs responsible for the Smithsonian tank, she stated to Frank M. Greco that frequent collecting trips were needed to keep the system stocked with live scleractinia.

The Pittsburgh AquaZoo also has a "reef" tank based on Dr. Adey's algal scrubbers. This tank is nothing more than a pile of rocks covered with filamentous green algae, and the water is QUITE yellow (as is the Smithsonian tank) from the presence of dissolved organics (ORP readings have been around 165). As with the Smithsonian tank, scleractinia do not survive longer than a few months. The same applies to soft corals as well. When I (Frank M. Greco) saw this tank on May 3, 1993, there were NO living corals to be found even though a collecting trip to Belize was made several months earlier and 81 pieces of living scleractinia were brought back. There were, however, two piles of dead Atlantic scleractinia: one right behind the tank and the other in the greenhouse housing the algal scrubbers.

The Carnegie Science Museum (Pittsburgh, PA) also uses an algal scrubber system, but with significant modifications. This tank looks the best of the three. There are several species of hardy Scleractinia and soft corals that are doing quite well. The water is clear (a bit cloudy). The major differences between this system and the other two is the use of carbon, a small, barely functioning algal scrubber, about 1000 lbs. of excellent quality live rock (Florida), water changes, and the addition of Sr and Ca.

The last system I know of that uses an algal scrubber is the Great Barrier Reef Microcosm in Townsville, Australia. As of this writing, the system is not maintaining live Scleractinia, and frequent collecting trips are needed in order to replenish the exhibit. It should also be noted here that while Dr. Adey has claimed in his book Dynamic Aquaria that corals have spawned in this system, what he doesn't mention is that the corals which spawned were collected only months before the known spawning season. From these few examples, it should be clear that algal scrubbers are NOT to be used in systems containing live scleractinia.

<Some theories, observations, and other comments withdrawn> The weight of evidence at this point seems to be against the use of algal scrubbing in reef tanks, and the method should be considered to be experimental. Beginners particularly are advised to avoid this technique until they have considerably more experience with reefkeeping. The advanced aquarist may well wish to experiment with this interesting and controversial method, but it would be unwise to risk the lives of an entire reef tank full of coral. Such experiments should progress slowly, beginning with the most hardy of inhabitants. Many of the objections center on stony coral survival, and it is possible that scrubbed tanks with fish and hardy invertebrates may do quite well.
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2.10 Live Sand

Of relatively recent interest in the hobby is the use of "live sand".   Live sand consist of small grain (0.5mm-1.0mm) coral sand that is * populated with crustaceans, worms, mollusks and bacteria. It is normally used at a rate of 10lbs per square foot of bottom area - which yields about a 1" deep covering. Variations from 1/8" to 3"s of covering have been reported.

If you decide to have a live sand substrate bottom, you should include several creatures that will turn-over, or otherwise, move the sand around. Recommendations include: Sea Cucumbers, Brittle Starfish, Serpent Starfish, Orange Spot and Golden Headed Sleeper Gobies,  Yellow Jawfish, Watchman Gobies, and other detritivores. A mix of the above is recommended, since each creature moves the sand around differently.

If you use sea cucumbers make SURE there is NO way one can enter into any pumps. If a cucumber gets stuck in a pump, it will potentially release extremly toxic substances into your tank. The only remedy is to start your reef tank over since no known anti-toxin exist. Yes, everything may die in your tank (strong skimming may save the day, but don't count on it).

Live sand has a reputation of eliminating the final traces of nitrates in otherwise well run tanks. It also provides an environment for additional bio-diversity in the tank. Additionally, some feel that the chemical balance and stability of a tank's water is improved when live sand is present.