Note that the pH remains stable for the first few additions of acid and then drops precipitously as the endpoint is approached. The initial stability of the pH reflects the fact that the solution (in this case seawater) is a buffer. Buffers are solutions that resist pH changes. This is why you will sometimes hear alkalinity or KH referred to as "buffering capacity."
The alkalinity of natural seawater is around 2.0 to 2.5 meq/L. To convert to the German degrees of carbonate hardness (dKH), multiply this number by 2.8, to yield about 6 to 7 dKH. It is generally recommended that a marine aquarium be maintained at an alkalinity somewhat higher than that of natural seawater, between 7 and 10 dKH. The higher alkalinity offsets the accumulation of acids typical of a closed-system aquarium. These acids come from several sources, the primary ones being carbon dioxide from respiration, nitric acid from biological filtration, and organic acids from metabolic wastes.
CALCIUM. Another dancer in the chemical ballet going on in the marine aquarium is calcium. Corals, crustaceans, mollusks, and calcareous algae all extract calcium from seawater, using it to construct their skeletons from calcium carbonate. Early efforts at maintaining the proper chemical environment for these organisms focused too narrowly on calcium alone, largely ignoring the role of alkalinity. This is unfortunate, because the availability of carbonate, the other essential component in skeletal structures, mostly depends upon the pH and alkalinity of the water. In fact, when the alkalinity is high, skeleton building can still occur, even when calcium is present at a level significantly below that of natural seawater. However, when both alkalinity and calcium concentration are low, corals do not thrive. Conversely, raising the calcium level above about 550 mg/L will result in precipitation of calcium carbonate as chalk, with a concomitant drop in alkalinity, and calcification is made more difficult. "Calcium hardness'' is the term used to describe the calcium content of the water, which for natural seawater is about 380 mg/L. Note that this is a simple weight/volume measurement; each liter of seawater contains about 380 mg of calcium. Maintaining this level is a relatively simple matter, as any soluble compound containing calcium can be added to the water to compensate for a deficit. However, remember from the discussion above that ions always come in pairs. Any compound containing calcium also contains a partner ion, and the nature of this partner can have important implications for aquarium chemistry. For example, using calcium chloride as a calcium source may result in the need for a "buffer additive" to restore the alkalinity. Sodium carbonate or sodium bicarbonate alone can be added to increase alkalinity, but better buffer additives contain other
128 Natural Reef Aquariums ions, such as borate, that participate in the alkalinity reactions of seawater.
Julian Sprung (1994) enumerated a number of benefits from adding limewater (or "Kalkwasser" in the parlance of German-speaking aquarists) to the aquarium, only one of which is to maintain the calcium concentration, but all of which have to do with the needs of the organisms being kept in the aquarium. Using limewater for calcium maintenance also helps to maintain the pH and alkalinity of the aquarium, because the hydroxyl ions from the limewater neutralize some of the acids accumulating in the system. In effect, this prevents the alkalinity from being "used up" and the pH therefore remains more stable. The ideal pH for calcification is about 8.40 to 8.45, and according to Sprung, aquariums should be maintained so that the pH does not rise above 8.45 during the day, except for a temporary rise (never more than 8.6) when a dose of limewater is added. Allowing the pH to go higher than this will, ironically, impede calcium maintenance, as calcium carbonate will spontaneously precipitate as the pH rises, once again robbing the system of both calcium and carbonate ions. Maintaining the proper balance is best accomplished through the use of an automated system for dosing the limewater, and an electronic pH meter. Adding the limewater by hand and evaluating pH with a color-change-type test kit can be done, albeit with more room for error.
The relationship between pH, calcium, and alkalinity is particularly noticeable in a tank devoted to macroalgae cultivation. Two opposing principles are at work in such a tank, and the combined result may explain why calcareous macroalgae, such as Penicillus, have been regarded as very difficult aquarium subjects. In the first place, these organisms remove calcium from the water profligately, compared to corals, owing to their much more rapid rates of growth. Secondly, photosynthesis carried out by macroalgae during the daylight hours also removes carbon dioxide from the water, driving the pH above 8.5. When the free C02 is used up, the macroalgae start utilizing bicarbonate ions. The overall result is a lowering of both calcium concentration and alkalinity and an increase in pH, all of which in-hib it the calcification process essential to the plants' growth. That this can happen in a matter of days in the confines of an aquarium makes one appreciate what an inexhaustible supply of calcium and carbonate ions are available in the ocean. In a reef tank containing many colonies of the pink and purple calcareous algae prized by reef enthusiasts, macroalgae growth can be self-limiting, even though conditions appear favorable.
Just as the problem of fine-tuning limewater additions can be aided through the use of a dosing system and an electronic pH monitor, so can the problem of too-high pH be met through the use of a carbon dioxide system controlled by an electronic pH controller. Such systems must be designed, however, to insure that overdosing the system with C02 does not happen should a malfunction occur. Carbon dioxide is toxic to fish in too high a concentration, and an excess will cause the pH of the tank to fall to disastrously low levels.
Besides aiding in the maintenance of pH, alkalinity, and calcium concentration, the addition of limewater to the reef aquarium has other benefits. Limewater addition, according to Sprung, also enhances protein skimming. This, he maintains, "promotes a cleaner, healthier aquarium environment, impeding the growth of filamentous and slime algae while encouraging the growth of calcareous algae and corals." According to Wilkens (1994), an important benefit of limewater addition is the near-total precipitation of phosphate (P04~3) from the water. Phosphate interferes with calcification because it is a "crystal poison" (see Simkiss, 1964, as referenced in Sprung, 1994) and has been shown to be detrimental to corals under natural conditions if present in excess (Burke, 1994). Also of great concern to
the aquarist, undesirable algae growth is almost always a consequence of excess phosphate. (Phosphate is a biolim-iting nutrient, meaning that algae cannot grow if starved for this nutrient. Interestingly, another biolimiting nutrient, nitrate, is always found in the sea in a precise 15:1 molar ratio with phosphorus. Not surprisingly, this is the exact ratio in which these nutrients are required by living organisms. Perhaps this observation helps to explain why limiting the phosphate, rather than the nitrate, concentration of the aquarium seems to be a more effective means of algae control. Due to its removal through chemical and biological processes, virtually no phosphate is found in the water around a coral reef.)
Other Factors Affecting Calcium & Alkalinity
We pointed out earlier that acid production is characteristic of biological filtration. Acid production depletes both calcium and alkalinity from the aquarium water, probably via the neutralization reaction and ion-pair formation. Systems, therefore, that accumulate nitrate will experience problems with maintenance of pH, alkalinity, and calcium hardness as well. The tendency to accumulate nitrate can be overcome by carrying out frequent partial water changes, a chore that many aquarists find especially bothersome, or by denitrifying the aquarium.
Denitrification is carried out by beneficial bacteria in several genera, including Bacillus, Denitrobacillus, Micrococcus, Pseudomonas, Thiobacillus, and others. These bacteria must have anaerobic (oxygen-free) conditions in order to carry out their chemical conversions. They are able to extract energy from the nitrate molecule, converting it to other chemical forms and, most importantly for our purposes, into nitrogen gas that ultimately escapes from the aquarium into the atmosphere. The overall reaction for this process is:
Note that six moles of hydrogen ions are used up for every mole of nitrate ions converted. As a result, the denitrification process also helps to retain alkalinity. In addition, denitrifiers consume carbon compounds (organic matter) to obtain energy. This process releases carbonates and bicarbonates, further enhancing the buffering system in the aquarium water.
Denitrifying bacteria probably reside within the interior of live rock. One of the most important aspects of the Berlin-style reef tank design is the reliance upon this source of denitrification. While many aquarists report low or even zero levels of nitrate in aquariums maintained exclusively by this technique, others report that in these systems, a stable, low level of nitrate, in the range of 10 to 20 mg NO3- per liter, is continuously maintained after the system has been operating for several months. But naturally occurring nitrate levels in coral reef waters are much lower than this, approaching zero, probably due to bioaccumulation. As a result, aquarists have sought methods of reducing nitrate concentrations to near zero levels without carrying out frequent or massive water changes.
Two different methodologies presently exist for enhancing denitrification. One approach, which has been a topic for discussion in the aquarium literature for years, is the use of a "denitrifying filter." Analogous to the wet/dry filter, this device seeks to optimize conditions for the growth of denitrifying bacteria. Recent versions employ electronic-control technology to monitor and adjust critical parameters. Hobbyists report that these units, though expensive, do work well.
The second approach has been closely investigated by Dr. Jean Jaubert in Monaco. In brief, this technique relies upon a thick layer of sand on the aquarium bottom to provide an appropriate home for denitrifying bacteria. A layer of aragonite sand is seeded with a layer of "live sand" collected from the ocean floor. Small organisms present in the
130 Natural Reef Aquariums live sand help to keep the substrate healthy, while denitri- therefore be prepared in small batches, used up within a fying bacteria thrive in the low-to-zero oxygen conditions. week, and any accumulation of CaC03 in the storage con-
It appears that this technique not only results in denitrifi- tainer should be removed periodically. To remove it, pour a cation, but also returns both calcium and carbonate ions to pint of water into the container and add 2 tablespoons of the water through the dissolution of the aragonite. Dr. white vinegar. Let stand 30 minutes, and the calcium car-
Jaubert has been successful in maintaining high levels of cal- bonate deposits should be easy to remove. Rinse well before cium and alkalinity in his systems, together with low levels of phosphate and nitrate, through reliance on these natural processes alone. These techniques are discussed more fully in Chapter Three.
Preparation & Use of Limewater
Caution! Calcium oxide (CaO) is somewhat caustic. Do not allow the dry powder to contact your skin, and keep it out of reach of children. For one gallon of limewater, place a rounded teaspoon (1.8 grams, to be exact) of calcium oxide Calcium hydroxide powder for in a clean, clear, glass or plastic container. making limewater (Kalkwasser). Fill the container almost to the top with
preparing another batch of limewater. (One can use this same technique to remove lime from other aquarium accessories or the outside of the aquarium glass. Just make sure that no significant amount of the vinegar solution gets into the tank.)
Using impure chemicals for making limewater can also lead to unintended results. Julian Sprung conducted experiments that bear this out. Some grades of calcium oxide (CaO) or calcium hydroxide (Ca(OH)2) may contain an excessive amount of magnesium, which, when added along with the limewater by the unsuspecting aquarist, results in calcium precipitation. Since magnesium is a component of the seawater buffer system, this distilled, reverse osmosis, or deionized water. Cap the con- situation will also be characterized by a very high alkalinity tainer and shake well. There should be some undissolved and high pH. Sprung s experiments indicated that the powder on the bottom of the container after it settles; if "magnesium effect" on the corals in his aquarium was posi-
not, add a little more calcium oxide and shake again.
tive for the first few days, and then things took a serious turn water.
Using "old" limewater can cause problems. As the so- for the worse, due to calcium depletion. The lesson here, of lution stands, it accumulates a layer of calcium carbonate course, is to use Ca(OH)2 of high purity for making lime-(CaC03), owing to a reaction with carbon dioxide from the atmosphere. Adding this material to the aquarium along The maintenance of pH, alkalinity, and calcium within with the limewater is a mistake, because the particles of their proper ranges is essential to the management of aquar-
CaC03 act as initiation sites for spontaneous crystalliza- ium water chemistry. A relatively new development is the tion of more CaC03, causing both alkalinity and calcium use of calcium reactors, in which aquarium water is contin-
to drop, which is just the opposite of the desired result of ually circulated through a column of aragonitic material or adding the limewater in the first place. Limewater should calcium carbonate into which carbon dioxide is injected.
Advocates say these reactors are easier to use than limewa- pH controller is essential for this much more sophisticated ter and do at least as good a job in maintaining both calcium system. Great stability can be achieved through the use of and alkalinity. A bonus claimed for these reactors is the such automated devices. Subtle, not drastic, fluctuations in leaching of other ions from the crushed coral gravel com- the composition of the water in the aquarium is one of the monly used, in theory providing minerals and trace elements marine aquarist's most important goals.
in close to the exact proportion required by growing corals. These units have been used for some time in Europe, apparently with outstanding results, but are largely untried in the American hobby as of this writing. The theory behind calcium reactors, which are becoming commercially available from a number of sources, appears sound, and these may prove popular, especially for systems with dense populations of growing corals, clams, or calcareous algae.
Until we have further experience with calcium reactors, however, the addition of limewater appears to be the time-tested, most satisfactory way to maintain a proper relationship among pH, alkalinity, and calcium. Using a A calcium reactor uses aragonite sand and system that employs a dosing carbon dioxide to enrich and buffer a reef tank, pump or that drips in limewater
Many aquarists use limewater to replace all evaporated water from the aquarium system, often adding about 1% of the system volume every day or two.
To summarize, limewater addition benefits the aquarium by:
1. Adding calcium ions.
2. Increasing alkalinity.
3. Precipitating phosphate from the make-up water and possibly from the aquarium water itself.
4. Increasing pH.
5. Facilitating protein skimming.
The ideal pH for the aquarium is 8.4 to 8.45. The ideal alkalinity is 7 to 10 dKH. The ideal calcium concentration is about
400 mg/L. From day to day, and even from hour to hour, these numbers will fluctuate, owing to the dynamic chemical and biolog-
continuously is a more suitable way of adding this supple- ical processes that are occurring in the aquarium continûment than simply pouring some in by hand. In any case, ously. Since the aquarium, unlike the ocean, is a small, adding limewater must be done while monitoring pH care- closed system, the aquarist must intervene to offset the fully. An electronic pH meter will greatly facilitate this pro- cumulative effects of these changes. This is the primary cedure, although satisfactory results can be achieved with a goal of the maintenance procedures explained in this book.
Proper maintenance of pH, alkalinity, and calcium con-
posed by employing automated additions of limewater while centration, especially in reef aquariums designed for stony pH is maintained by carbon dioxide injection. An electronic corals, has resulted in intense interest among hobbyists in good-quality test kit. A tighter level of control can be im
132 Netural Reei Aquariums additives and techniques that can be used to replenish important ions and stabilize pH. Two worth investigating are: a two-part liquid product said to supply calcium ions and boost alkalinity without affecting pH, and the German technique of passing distilled water over calcium carbonate to produce a solution that is added to the aquarium. Walter Adey (Adey and Loveland, 1991)
described a carbonate reactor, similar to the German design discussed in the previous section, that employed skeletal fragments of the calcareous alga Halimeda as a source of calcium carbonate. Aragonite, the most soluble crystalline form of calcium carbonate, can be utilized in Monaco-style denitrifying sand beds, as well as in a carbonate reactor. In either application, dissolution of the aragonite returns both calcium and carbonate ions to the water.
The critical parameter for calcification by corals is a stable, high pH, with the optimum being about 8.4. Invertebrates expend energy to pump calcium actively from the water and can make do with levels of calcium that are lower than normal if pH and alkalinity (>2.5 meq/L) are correct. Once an equilibrium is reached, adding calcium ions doesn't do much, because excess calcium combines with carbonate and restores the previous level. The natural level for calcium is often reported as a litde higher or a little lower than the generally accepted 400 mg/L, depending on how the measurements were made. Too many aquarists strive for exactly 400 mg/L of calcium, while neglecting the more critical parameters of pH and alkalinity.
Another important point in calcium testing: Assuming your reading is 400 mg/L using a titration test kit, you must allow for a fairly wide experimental error. If, for example, the smallest increment that can be discerned (one drop of titrant) represents 20 mg/L of calcium in the test kit you are using, your actual reading is 380 to 420 mg/L (400 ± 20
mg/L). One way to even out such errors is to perform the test in triplicate and average the results. There are also more accurate methods of carrying out titrations than those provided by test kits, but none are as practical for home use.
Phosphates & Algae Control
Compounds containing phosphorus and oxygen are called phosphates and occur in all living organisms. Phosphates find their way into aquarium water as a result of various biological processes and are also introduced by foods and as an impurity in some chemical products sold for aquarium use. Accumulation of phosphates is one of the major factors in the growth of annoyingly overluxuri-ant algae mats that may sometimes occur in a marine aquarium. Algae become a "problem" when they interfere with the aesthetic appeal of the aquarium or when rampant growth threatens to smother delicate organisms like sessile invertebrates. There is no simple means of controlling excess algae growth, because a variety of factors are involved.
In most marine aquarium situations, the limiting nutrient ion, in terms of algae growth, is phosphate. Phosphate limitation is therefore the single most effective means of algae control, but presents a problem from a practical point
Popular two-part additive with calcium solution, alkalinity supplement, and other seawater ions.
of view. The first step in eliminating phosphate from the aquarium is to introduce as little of it as possible to begin with. This is challenging, since phosphate is ubiquitous in nature and enters the aquarium from a variety of sources: all foods, some salt mixes and tank additives, and especially tap water. My city's tap water contains 0.1 ppm phosphate on an average day. This is about 1,000 times the concentration found in tropical ocean water. Using water that is more purified than tap, well, or spring water has worked wonders for many aquarists who were formerly plagued with algae. One of the easiest ways to purify water at home is by reverse osmosis (RO). An RO unit will produce 90% pure water for only pennies per gallon. Keeping the aquarium clean by removing accumulated detritus on a regular basis will help to lower phosphates. When aquarium water has 1.0 ppm phosphate or less, a phosphate-remov-ing material can be used to reduce the concentration. (Typically, a phosphate-absorbing sponge or granular resin in a mesh bag is placed in the filter system so that tank water can pass over or through it.) Some of these products receive commendable ratings from experienced reef keepers, but if phosphate concentrations are above 1.0 ppm, changing water (using seawater mix prepared with purified water) is a more practical solution for getting an elevated phosphate level back under control.
Ideally, there should never be any phosphate that is measurable with a test kit that uses the ascorbic acid method — the only method that works well in seawater — and is ac curate to 0.05 mg/L. If such a test shows any measurable level of phosphate, one should start looking for ways to reduce the phosphate concentration in the tank. Mix up some fresh synthetic seawater as if for a water change (use purified water). Test this for phosphates. There should be none. Mix up a bucket of purified water with the appropriate dose of any additives you may be using and test it. There should be no phosphate here, either.
One final note about testing for phosphates. Purchase a supply of disposable plastic vials in which to carry out phosphate tests. An aquarium dealer, druggist, or a chemical-supply house should carry these. Use a fresh vial for each test. Phosphates are difficult to remove from glass and plastic and can build up and result in spurious test results. To avoid this, I like to use a new vial each time a phosphate test is performed.
To understand how and why excessive microalgae growth occurs in the aquarium, one can begin by asking why natural reefs do not become overgrown with microalgae. There appear to be several reasons. First, on a natural reef, much of the space available for colonization by microalgae is already occupied by macroalgae or encrusting animals.
These organisms possess a variety of adaptations to prevent their living space from being usurped by competing organisms. This is one reason why I advocate the use of live
Growing calcareous algae, such as this Halimeda, and many corals can rapidly deplete calcium levels in the aquarium.
ALGAE: THE GOOD, THE BAD, AND THE MERELY UGLY
So much commotion has been raised over algae control in the aquarium that beginners get the notion that the presence of algae is "bad." On the contrary, algae are important components of the aquarium ecosystem. But just as both roses and dandelions may grow in the same garden, some algae are more desired by marine aquarists than others. Calcified algae found in shallow lagoon habitats, such as Hal-imeda, Penicillus, Udotea, and Cymopolia, are striking specimens in their own right and require conditions similar to those needed by stony corals. Caulerpa, a vinelike genus found on rubble, sand, and mud, grows very rapidly and therefore needs frequent pruning to remain in bounds. It also leaks yellowing compounds into the water, but this can be counteracted through protein skimming and, if necessary, activated carbon filtration. The prunings can be fed to herbivorous fish. Caulerpa growing in a shallow-water tank with large soft corals produces a natural-looking display. From deeper waters, and consequently tolerant of less intense lighting, red algae grow less abundantly than green species and can be included in a variety of reef habitats. Gracillaria, cultivated as food in some parts of the world, is easily grown and looks pretty with mushroom coral polyps. Botryocladia is often seen on rock specimens from the Gulf of Mexico and Florida Keys and looks like a bunch of red grapes.
Among the undesirable types of algae that may crop up in the aquarium is Valonia, which looks like a mass of green bubbles. It spreads, sometimes aggressively, even overgrowing corals. Maintaining a high pH and removing the bubbles wherever possible are the only means of control, although a bloom usually subsides on its own. "Hair algae" is a catchall term for filamentous forms such as Cladophora, Cladophoropsis, and Derbesia. These rapidly growing algae develop — often on bare, dead rock surfaces — in response to high nutrient levels. Bryopsis, a bushy, feathery dark green species reaching about 1 inch in height, may bloom in aquariums with too many nutrients and too little water movement. Golden-brown, oily films or strings of diatoms result from water with high silica content. These microalgae will always be present, blooming when nutrient levels rise. Dinoflagellates, another variety of golden-brown algae, may also undergo periodic blooms, often in reef aquariums that have been set up for a while. Fortunately, when good conditions are maintained, algae blooms are rare, and those that do occur typically run their course in a few weeks' time.
Valonia species (Bubble Algae, Sea Pearls) at times become a nuisance in nutrient-rich systems.
rock for construction of the entire "reef" within the tank. Some aquarists have chosen to use dead coral rock, lava rock, and similar products to build a base and then add live rock on top. In my experience, this practice will often encourage colonization of the bare rock surfaces with microal-gae that can grow and spread at a much faster rate than any of the organisms present on live rock are able to do. Thus, before the dead rock material can become encrusted with desirable organisms, microalgae gain a foothold.
A second reason for the ack of microalgae on natural reefs is the abundant presence of herbivorous (algae-eating) animals. Many species of worms, mollusks, echino-derms, crustaceans, and fishes are constantly nibbling away the growths of algae that develop. Certainly, the addition of some of these organisms to the reef aquarium can benefit in achieving the overall goal of algae control. That these two factors — lack of colonization sites and predation by herbivores — can account for the absence of huge mats of microalgae from natural reefs is supported by observations of what happens when these factors are naturally eliminated. On the reef crest, where scouring by pounding waves prohibits coral growth and light is intense, microalgae are often the dominant organisms. Herbivorous fishes favor the reef crest because of the abundant food supply. A balance is achieved between the growth rate of the algae and removal by herbivores — if an area is picked clean, the herbivores will simply graze elsewhere, permitting regrowth of the algae mats to occur. Certain damselfishes will stake out a particular coral head and then systematically kill a portion of the living coral by nipping at the polyps. The dead coral skeleton rapidly becomes colonized with filamentous algae. The damselfish will vigorously defend its home, driving off intruders much larger than itself, including fishes that would feed on the algae growth. As a result of this behavior, the damselfish is able to cultivate a private garden of filamentous algae, which it eats, thereby gaining energy and important nutrients without having to stray from the protective confines of the coral head. Because the damselfish feeds judiciously and supplements its diet with other organisms, such as plankton that it plucks from the water column, the growth of its garden of microalgae keeps pace with its food requirements. Here we have a natural analog to an aquarium in which only dead corals and coral rock are used as decorations and from which herbivores are absent.
Not all aquariums are devoid of live rock and herbivorous animals, yet they still experience blooms of microalgae. So other factors must be involved. These have to do with water chemistry, specifically the presence of nutrient ions. The ocean around natural coral reefs is characterized by very low concentrations of nutrient ions, in contrast to the waters of the sea in many other areas. Nutrient ions are necessary for the growth of all organisms and include nitrogen compounds, carbon compounds, and phosphates. Nitrogen compounds in seawater include mostly ammonia, nitrite, and nitrate. Marine aquarists have always made every effort to keep ammonia and nitrite concentrations at or near zero, but only recently have they become aware of the importance of keeping nitrate concentrations near zero as well. Measured values for total inorganic nitrogen in tropical seas are typically in the range of 1 to 2 micrograms per liter. That's 1 to 2 parts per billion for the sum of all ammonia, nitrite, and nitrate present. For the aquarist using simple test kits, this number is effectively zero. The concentration of phosphorus compounds (measured as dissolved inorganic phosphorus, or orthophosphate) in tropical oceans is even lower than the concentration of nitrate, often only 1 to 2 parts per 10 billion. Again, this is "zero" for aquarium purposes.
In the case of carbon compounds, consider the various forms in which carbon may be present. Carbon dioxide gas is dissolved in the aquarium water from the atmosphere and also occurs as a result of the respiration of the organisms
136 Natural Reef Aquariums in the tank. C arbon is also present in the form of carbonates, ions that participate in the pH buffering system and are measured by an alkalinity test. The third form of carbon is dissolved organic carbon (DOC). Measuring the level of DOC in seawater is difficult to carry out, but values that have been reported range up to a maximum of about 2 parts per million. Control of microalgae growth in the aquarium consists primarily in keeping the concentrations of nitrate, phosphate, and DOC as close to zero as possible.
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