A hallmark requirement of reef systems is their water quality; it must be optimized and stable. There are many ways to achieve this mythical "high water quality". For folks fortunate by geography there are open systems, pumping new water from the sea and overflowing old to waste. For most all aquarists however, the sum total of what we do to preserve and improve our recirculated water is termed filtration.
There is no absolute right/wrong to any given arrangement of components, makes and models of gear in a filtering scheme. Many methods are employed with greater/lesser success that often has more to do with diligent testing, observation and maintenance than cost and complexity of gear. Bear in mind that whatever techniques employed, the goal is the same... no detectable ammonia, nitrites; low nitrates (less than 10ppm), low phosphate (PO4, less than 0.05 mg/l), high redox (up to 425), elevated calcium (ideally 400-475 mg/l), pH 8.0-8.4, alkalinity 2.5-5 meq/l, near-saturation oxygen (generally about 7 mg/l)...
Here is a brief review of filtration types employed for reef use, with my comments on their suitability, and current preferences:
Days of Yore: The Under-gravel Filter, With and Sans Power
Starting in the 1950's and persisting in some places to this day, is the typical "fish-only" marine setup method employing the lowly under-gravel filter, possibly accompanied with outside power filtration. Coupled with plenty of water changing, adequate lighting and low stocking density this low-tech. approach can work satisfactory for the toughest of reef organisms. As far as creating and maintaining optimized water quality, air and power-head driven systems are low-scorers. Such "nitrate-factories" are better adapted for use as low oxygen denitrators by increasing substrate depth and removing their airlift tubes. They attain their highest and best use when employed in a totally separate sump or refugium where periodic cleaning and other disruption does not disrupt activity in the main/display tank.
Dutch/Wet-Dry/Trickle Filtration: Next
During the late seventies, up through the 1980's and continuing with refinements, the modified sewage treatment technology was/has been/is a great leap forward in improving and maintaining better water quality. To a large degree, these units not only herald, but brought on the mini-reef revolution in aquarium keeping.
Such systems employ a wet and dry area above a separate sump to promote the activity of nitrifying microbes, outside of the disturbances of the main tank. The trickled outflow is well-oxygenated and out-gassed and often treated mechanically (sponge, cartridge, foam) and physically (ozone, U.V.) before being returned to the main via a small centrifugal pump.
To their discredit, most types of wet-dries suffer the same complaints as their predecessor undergravels; fluctuating pH, loss of alkaline reserve, excessive nitrate production. Better than all the wacky high-priced moist media, came the next wave in reef filtration, named in honor of its German heritage.
Berlin-Style Filtration: Live Rock & Protein Skimming
Either take the above prototypical wet-dry systems media out entirely or start with a bare tank, add a large quantity of cured live rock to aforesaid wet-dry sump or gravel-less tank, toss in an efficient protein skimmer and you have the essence of a Berlin-Style filtered set-up. Further commonalities of such arrangements are intense lighting (generally metal halide and actinic fluorescents), and the chemical feeding of limewater (and possibly other minerals) typically as a part of daily evaporation make-up.
Do they work? Yes; the microbes living in and on the rock readily process wastes, aerobically and without the benefit of oxygen. Protein skimming removes most remaining organic matter, and more, especially "color", may be removed with the use of activated carbon.
More of an adjunct additional to other filtration forms, the "jaw-bare" (after Dr. Jean Jaubert) or Monaco Aquarium method of denitrification (after its popularizer and his employer) utilizes a thick bed of calcium carbonate substrate, a raised grid of some sort (undergravel filter plate, eggcrate covered with fiberglass cloth...), and a low oxygen plenum zone of water within to promote the anaerobic chemical and biological reduction of nitrates. Some folks add a further screen over a portion of the substrate to keep the above-gravel livestock from digging too deep.
These low maintenance units are great not only for driving down the accumulation of the "end products" of aerobic nitrification, but also as harbors and cultures of interstitial fauna that can add a great deal of reef fish nutrition. To top it all off, the reductive nature of the denitrator dissolves the calcareous sand, making minerals like strontium and magnesium available to corals, clams and more.
What's Hot: A Mix of Techniques
The late nineteen nineties find that as with human affairs, "there are no great leaders, just great committees"; what is being most utilized is a hodge-podge of the best of all the above (and more) filtration ideas, methods and attitudes. High quality live rock, live sand with or without the Jaubert plenum arrangement, vigorous circulation, and improved protein skimming are de riguer. Often, where space and budgets allow, aquarists are discovering the benefits of alternating light/dark cycles on a separate sump/refugium where more live rock (LR) and possibly live sand (LS) are remoted.
All this of course with the requisite intense lighting, testing, adjustment and supplementation that is required or desired for boosting deposition and growth of demersal invertebrates.
Other: The Big Category
Oh yes, there is so much more. Algae scrubbers of different sorts, fluidized beds utilizing sand, glass beads and other media; sintered glass, specialized resins and other novel material placed in the system or in-line with the external filtration/circulation array.
Filtration, en toto, embodies all you can do to mimic or even best "natural conditions": low organics, high oxygen tension, ready concentrations of necessary minerals and foods; non-linear, chaotic and robust water movement. All at the lowest cost , maintenance and potential for breakdown. What will be the next big improvement in reef filtration? Maybe you'll be the design/engineer, or dreamer who comes up with it.
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