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You are here: Home Volume II October 2003 Aquarium Chemistry: Magnesium In Reef Aquaria

Aquarium Chemistry: Magnesium In Reef Aquaria

By Randy Holmes-Farley, Ph.D. Posted Oct 14, 2003 08:00 PM Pomacanthus Publications, Inc.
This article details the nature of magnesium in seawater, how it is added, measured, and removed from marine aquaria, and how it impacts the maintenance of calcium and alkalinity.

Magnesium is the third most abundant ion in seawater, behind sodium and chloride. It is also intimately involved in a great many biological processes in every living organism. Nevertheless, the only time that it comes to the attention of most reef aquarists is when it is suspected of causing a problem in maintaining appropriate calcium and alkalinity.

This article on magnesium is the first of several that delve into a variety of issues involving magnesium and strontium. Calcium, magnesium, and strontium are very similar chemically. So similar, in fact, that they get in the way of each other in a variety of situations, and that is part of the reason why these ions merit interest by aquarists. This article details the nature of magnesium in seawater, how it is added, measured, and removed from marine aquaria, and how it impacts the maintenance of calcium and alkalinity.

Future articles will cover some of these same issues for strontium, and will also explore the delivery of these ions to aquaria in supplements of various sorts. How much magnesium and strontium are delivered to aquaria in limewater (kalkwasser), for example, is not at all obvious. In fact, it almost certainly depends on exactly how the limewater is prepared and delivered. These issues will be addressed experimentally on the actual products that many aquarists use.

Magnesium in Seawater

In full strength seawater (S=35), magnesium is present at approximately 53 mM (mM is short for millimolar, which is a measure of the actual number of ions present, as opposed to ppm (parts per million), which is a measure of the mass of ions present). Only sodium (469 mM) and chloride (546 mM) are present at higher concentration, with sulfate (28 mM) following close behind. Magnesium is about five times more abundant than calcium (10 mM). Magnesium is significantly lighter than calcium, so when compared on a weight basis, it is only about 3 times as concentrated (1285 ppm vs. 420 ppm).

chem_figure_1_thumb.jpg

Figure 1. This sally lightfoot crab probably doesn't realize that the coralline algae that it is standing on likely has a high magnesium content. Photo by Debi Coughlin.

One other comment on seawater concentrations of magnesium. The magnesium content of seawater has not been constant since the oceans formed. Specifically, the magnesium content has often been lower, as in the late cretaceous period. As is discussed below, the amount of magnesium getting into calcium carbonate skeletons is a function of how much magnesium is in the water. Consequently, the magnesium content of ancient sediments can be significantly lower than more modern ones from similar organisms.1 In addition to being an interesting fact, this result may also play a role in the suitability of certain limestone deposits in maintaining magnesium in aquaria. For example, such limestone is sometimes used in CaCO3/CO2 reactors or as the raw material for making calcium hydroxide (lime). If it is low in magnesium, one may find additional supplements necessary to maintain modern seawater magnesium concentrations. These issues will be detailed more in future articles.

Magnesium is present in seawater as the Mg2+ ion, meaning that it carries two positive charges, just as calcium does. Most of the magnesium is present as the free ion, with only water molecules attached to it. It is estimated that each magnesium ion has approximately eight water molecules tightly bound to it. That is, water molecules that are so tightly bound that they move with it as the magnesium ion moves through the bulk of the water. For comparison, singly charged ions like sodium have only 3-4 tightly bound water molecules. A small portion (about 10%) of the magnesium is present as a soluble ion pair with sulfate (MgSO4), and much smaller portions are paired with bicarbonate (MgHCO3+), carbonate (MgCO3), fluoride (MgF+), borate (MgB(OH)4+), and hydroxide (MgOH+).

While these ion pairs comprise only a small portion of the total magnesium concentration, they can dominate the chemistry of these other ions. An extended discussion of these facts is beyond the scope of this article, but is should be noted that these ion pairs can have huge impacts on seawater chemistry. In the case of carbonate, for example, the ion pairing to magnesium so stabilizes the carbonate that it is present in far higher concentrations than it would be present in the absence of magnesium. This effect, in turn, makes seawater a much better buffer in the pH range of 8.0-8.5 than it otherwise would be. Without this ion pairing, seawater pH might be significantly higher, and more susceptible to diurnal (daily) swings.

The average residence time for a magnesium ion in seawater is on the order of tens of millions of years. That time is substantially longer than that for calcium (a few million years) and aluminum (100 years), but less than sodium (about 250 million years). In a certain sense, this is an indication of how reactive magnesium is: it stays in seawater a long time because it's fairly unreactive, but it does get taken out of solution through various biological and chemical processes more readily than does sodium.

Another interesting characteristic of ions is whether they are excluded from organisms, actively taken up, or just "allowed" to be present. Like two other common ions, sodium and sulfate, the relative concentration of magnesium in organisms is approximately the same as in seawater (not counting magnesium in skeletons). This probably results from the fact that there is plenty of magnesium present in seawater, and that it is used by organisms for many purposes. Chloride, another very common ion, is actively rejected by organisms, and most other ions are substantially concentrated.

Organisms That Use Magnesium

In terms of the amount of magnesium consumed, the primary use in reef aquaria is in calcification. When calcium carbonate skeletons are deposited, magnesium often gets into the skeleton in place of calcium. It is not entirely clear whether this is something that organisms "try" to control or not. Nevertheless, the amount of magnesium entering the skeletons of different organisms varies greatly. Table 1 shows the relative amount of calcium and magnesium in calcium carbonate skeletons of various organisms.

Table 1. Magnesium in calcium carbonate skeletons
Organisms Magnesium content of skeleton (weight %) Reference
Corals
Suborder Asterocoeniina and Faviina 0.07 - 0.36% 2
Suborder Fungina 0.095-1.22% 2
-- Fungia actiniformis var. palawensis 0.091% 6
Suborder Caryophylliina 0.18-0.21% 2
Suborder Milleporina 0.12-0.53% 2
-- Millepora sp. 0.12-0.53% 2
Suborder Stolonifera 2.98-3.52% 2
-- Family Tubiporidae 2.98-3.52% 2
-- -- Tubipora rubrum 2.98-3.52% 2
-- Family Dendrophylliidae 0.05% 2
-- Family Porites 0.095-1.22% 2
-- -- Porites lobata 0.40-1.22% 2
-- Family Pocillopora 0.34% 2
-- Family Dendrophyllia 0.05% 2
Gorgonia
Eunicella papillosa, E. alba, E. tricoronata, and Lophogorgia flamea 2.2-2.7% 5
Other Organisms
Coralline Algae in general >1% 1
Coralline algae: Lithophyllum and Lithotamnium 2.0-2.8% 7
Calcareous alga Corallina pilulifera 4.4% 4
benthic marine Ostracoda (crustaceans) 0.5-1.3% 3

Interestingly, coralline algae that normally packs a large amount of magnesium into their calcium carbonate deposits (>4 mole percent magnesium carbonate, or >1% magnesium by weight) has been shown to incorporate less magnesium when the magnesium content of the water is reduced. The amount incorporated is directly proportional to the magnesium concentration. Consequently, the amount of magnesium that they consume in aquaria is dependent on the magnesium content of the water. This effect is also likely to extend to other calcifying organisms as well.1

In addition to that used in calcification, many organisms (if not all) take up magnesium from seawater. Organisms ranging from bacteria8-10 to fish11 take up magnesium. In many cases, there is so much magnesium in seawater that the organisms need to spend more effort pumping back out excess magnesium than they do trying to take it up. For example:

"That the kidneys of marine fish have powerful renal mechanisms for the excretion of magnesium (Mg) from the body has been known since the early 1930s…"11

Toxicity Of Elevated Magnesium

There have been very few studies on the toxicity of elevated magnesium on most marine organisms. Most toxicity studies involving magnesium use freshwater species. This is largely true because magnesium is already quite high in concentration in normal seawater, so to significantly elevate it requires conditions that would rarely be encountered in oceans or even lagoons.

Bingman12 pointed out in a previous article that at elevated concentrations (>8,000 ppm), magnesium has been used as an aid in shucking oysters, helping to force the oyster open,12-14 and also as an anesthetic for them.12 Consequently, magnesium does have potentially negative biological effects at significantly elevated concentrations.

Toxicity of Depleted Magnesium

Like elevated magnesium, there are not very many studies on the effects of depleted magnesium on creatures likely to be present in aquaria. Except in estuaries and at the outflow of hydrothermal vents, magnesium is not likely to be depleted in marine systems. Consequently, few scientists have much interest in studying such depletion. It is known that many marine bacteria do require magnesium, but in some cases, just a little magnesium is adequate.8-10 On first principles, all autotrophs (organisms that get all of their energy from photosynthesis, including all algae) must get their required magnesium from the water column. How high the magnesium concentration needs to before they become limited by magnesium, however, is not known.

Magnesium in Marine Aquaria

Magnesium has tremendous biological and chemical relevance to reef aquaria. Fortunately for reefkeepers, it is present in abundance in seawater. There is, in fact, a fairly high turnover of magnesium in reef aquaria with rapidly calcifying organisms. The primary reason that magnesium is not more of a daily concern to aquarists is that the reservoir of magnesium in seawater is very large. Magnesium might be compared to a large lake, with the lake level only slowly responding to changes in inputs from rivers and export via evaporation and the outlet. Consequently, maintenance of magnesium levels is not typically a rapidly developing problem. If using an appropriate salt mix, it may never become a problem for many aquarists. Nevertheless, over the long run the levels can change significantly if the inputs and exports do not roughly match. The following sections will describe these inputs and exports, and will also describe what happens if the magnesium gets too low.

Sources of Magnesium in Marine Aquaria

The obvious primary source of magnesium in aquaria is the artificial or natural seawater used to set up the aquarium, and with which any water changes are performed. Some artificial salt mixes have been reported to be deficient in magnesium.15 These include Tropic Marin and Seachem. Others have been reported to have a substantial excess, including Coralife.15

The other major source is calcium supplements. Many of these supplements contain magnesium, either by "accident" (as in the case of calcium carbonate with impurities of magnesium carbonate that is used in CaCO3/CO2 reactors) or because magnesium is intentionally added by manufacturers.

In the case of commercial calcium supplements, some manufacturers add magnesium to some of them. Seachem, for example, adds magnesium to Reef Complete and Reef Advantage Calcium, but not to their other calcium products. When formed into the equivalent of a calcium carbonate skeleton, the amount added is equivalent to about 2% by weight magnesium in the "skeleton". As will be seen below by comparison to other methods, that is fairly high. How much is an optimal amount to add, however, is not entirely clear, and this "problem" of matching the input of magnesium to the export is discussed in detail below. Other manufacturers, such as Kent, do not add magnesium to any of their normal calcium supplements. Nevertheless, these supplements will all contain some magnesium. The question is how much.

In some cases, most notably that of limewater (kalkwasser), it is not clear that all of the magnesium present in it actually makes it into aquaria. Magnesium hydroxide may settle from solution prior to addition to the aquarium. Consequently, even though analyses of commercial lime products show fairly large amounts of magnesium (the quicklime that I use would produce a calcium carbonate skeleton with approximately 1.8% magnesium by weight),16 how much makes it into aquaria is likely a complicated function of how much lime is added to how much water, how long it is allowed to settle (if at all), and whether any vinegar is added into the milieu. These issues will be explored experimentally in a future article.

Even when the included magnesium is nearly all getting into the aquarium, as in a CaCO3/CO2 reactor, striking a perfect balance between input and export may require occasional measurements and adjustments. The argument that using ground up coral skeletons in a CaCO3/CO2 reactor will supply exactly what corals need is too simplistic. Different sources of calcium carbonate have different amounts of magnesium in them. In testing of samples used by aquarists, Bingman18 reported 0.1% by weight magnesium for Korlith and 0.28% magnesium for Super Calc Gold. Similarly, [Hiller][29]18 reported 0.4% by weight magnesium for a quarried limestone and 0.26% magnesium for Nature's Ocean brand crushed coral.

Further complicating the lives of aquarists is the fact that different organisms use different amounts of magnesium relative to calcium (Table 1, where the organisms are seen to range from 0.05% to 4.4% magnesium by weight in the skeleton). Consequently, the optimal amount of magnesium to provide to an aquarium, relative to calcium input, is going to depend on exactly what organisms are in the aquarium. For this reason, one may find that simple use of any particular calcium and alkalinity supplementation scheme may lead to magnesium declining (or rising) over time. These issues will be addressed more extensively in future articles.

Another potential source of magnesium is fish food. Magnesium is present in many such foods at fairly high concentrations, but not enough to have a significant impact on typical levels of magnesium (~1285 ppm). Table 2 shows some data from Shimek19 that have been recalculated to show the effect of adding 5 grams of food per day to a 100-gallon aquarium for a year. The effect on magnesium of 1-14 ppm assumes that all of the magnesium goes into solution. Whether that actually happens or not is moot as the total contribution to magnesium is small. Also shown in Table 2 are the same data for calcium, showing that some foods could add a significant amount of calcium to reef aquaria.

Table 2: Calcium and Magnesium in Aquarium Foods
Food Calcium Concentration (ppm) Calcium added to 100 gallon tank in 1 year (ppm in aquarium) Magnesium Concentration (ppm) Magnesium added to 100 gallon tank in 1 year (ppm in aquarium)
Formula One 800 4 280 1
Formula Two 1700 8 290 1
Prime Reef 860 4 290 1
Lancefish 4700 23 520 3
Silversides 4300 21 560 3
Brine Shrimp 140 1 300 1
Plankton 1700 8 520 3
Golden Pearls 8700 42 920 4
Gold Flakes 7200 35 1300 6
Tahitian Blend 440 2 290 1
Saltwater Staple 17000 82 1800 9
Nori 2400 12 2900 14

Sinks for Magnesium in Marine Aquaria

The primary sink for magnesium in aquaria is coprecipitation with calcium carbonate. This occurs in organisms, as shown in Table 1, and also during the abiotic (non-biologically driven) precipitation of calcium carbonate (such as on heaters).

A potential sink that has been described by some hobbyists is the precipitation of magnesium by limewater (kalkwasser). Both magnesium hydroxide and magnesium carbonate have been suggested. I do not believe that either is an important process in most aquaria. Adding any high pH additive, including limewater, results in the transient formation of magnesium hydroxide. This material quickly redissolves on mixing such that the local pH drops below about 8.6.-9.0. Magnesium carbonate is a more complicated issue, as it is near its solubility limit in seawater and may quickly get coated with a less soluble magnesium calcite. These issues have been dealt with by Bingman20 in much greater detail, and his conclusion is that neither of these precipitates is a likely sink for magnesium.

I would suggest that an alternative way that aquaria using only limewater might become deficient in magnesium over time is that the limewater is simply not delivering magnesium to the aquarium even though it is present in the solid lime. How and why this might happen was discussed above involving the precipitation of magnesium hydroxide in the limewater reservoir. This lack of addition coupled to the ongoing removal of magnesium in calcification could lead to deficiencies in magnesium. Such deficiencies have not become extensive in my aquarium, but it does not have an especially high rate of calcification, and perhaps water changes have eliminated the problem. In any case, those using only limewater (or other systems that do not deliver magnesium) may want to occasionally check magnesium.

Supplements for Magnesium in Marine Aquaria

There are a variety of commercial supplements for magnesium. Those supplements made by ESV, Seachem, and Kent are quite popular, although I've not seen any detailed analyses of them. Assuming they are what they claim to be, they are fine products to use, even for large increases in magnesium. I've used the ESV supplement, along with ones that I've made myself.

One thing to keep in mind about magnesium supplements is that they are all necessarily quite "dilute" even when presented as dry solids. The reason for this is that magnesium is a doubly charged and very light ion. So in a salt form, or when dissolved in a liquid, it is necessarily attended by a large number of quite heavy counterions (chloride and sulfate, especially). Commercial dry supplements may be only 8% magnesium by weight, for example.

Compounding the issue is the simple fact that there is so much magnesium in an aquarium that significant supplementation requires a great deal of material. A 100-gallon aquarium contains about a pound of magnesium! In order to raise that same aquarium by 200 ppm of magnesium, one would need to add on the order of 2 pounds of dry magnesium salts!

Epsom Salts (USP grade magnesium sulfate heptahydrate) is readily available in drug stores and very inexpensive. The problem is that if you were to raise magnesium by a large amount (or a small amount several times) the aquarium water will become relatively enriched in sulfate. This enrichment may not be a problem for some aquaria, especially those using salt mixes already deficient in sulfate,15 or those that experience frequent water changes. Bingman21 has addressed these enrichment issues and has suggested a recipe for home made supplements based on Epsom Salts and magnesium chloride. The problem is in getting the latter in adequate purity.

As an alternative, some aquarists have begun to use Nigari, a Japanese product that is derived from seawater and is used to manufacture tofu. It appears to be mostly magnesium salts of chloride and sulfate, but how much sulfate and how much chloride, as well as what other metals remains to be demonstrated.

Whatever supplement you choose, I'd suggest targeting the natural seawater concentration: 1285 ppm. For practical purposes, 1250-1350 ppm is fine. I would not suggest raising magnesium by more than 100 ppm per day. If you need to raise it by several hundred ppm, splitting the addition over several days will allow you to better home in on the target concentration, and might possibly allow the aquarium to deal with impurities that may come in with the supplement.

It has been suggested that adding dolomite to CaCO3/CO2 reactors can help with magnesium problems. Dolomite is a material that contains both magnesium and calcium carbonate. If dolomite is being added to the reactor to maintain existing appropriate magnesium levels against the continual depletion via calcification (for example, if the calcium carbonate being used is too low in magnesium to maintain adequate magnesium) then this is a fine approach.

However, this method is unsuitable if the goal is to raise magnesium levels. The problem is that for every magnesium ion released from the dolomite, 2 units of alkalinity are also released:

MgCO3 → Mg2+ + CO32-

Consequently, if one wants to raise magnesium by 100 ppm, the alkalinity will necessarily rise by 8.2 meq/L (23 dKH). The only way around this problem is to add a mineral acid (not vinegar) to the aquarium to reduce the alkalinity, and that may be more problematic than just adding magnesium in the first place.

Measuring Magnesium in Marine Aquaria

There are a number of commercial magnesium test kits available, including those made by Hach, Salifert, and Seachem. These kits have various ways of trying to distinguish magnesium from calcium. Some, like the Hach kit, require 2 titrations, but you end up with magnesium and calcium readings. Others, like the Seachem kit, require only a single titration, but yield only a magnesium value when used that way. In most cases, simply following the manufacturer directions will get you what you want, but the units are sometimes complicated. The Hach kit, for example, reports magnesium in units of calcium carbonate equivalents! While this facilitates the subtraction process used in the Hach kit to arrive at a magnesium value, it also serves to confound many aquarists. Hopefully future versions of that kit will strive to make the units more clear. For those interested, Bingman22 has a more detailed discussion of the chemistry involved in these kits.

Effect of Magnesium on the Calcium/Alkalinity Balance in Aquaria

How does magnesium impact the balance of calcium and alkalinity23 in reef aquaria? In order to answer that question, one has to have a basic understanding of the calcium and carbonate systems in seawater. These systems have been discussed in detail in a variety of previous articles, so I won't go into them here in great detail. In short, calcium carbonate (CaCO3) is supersaturated in seawater,24 meaning that given enough time calcium ions will interact with carbonate ions and precipitate as calcium carbonate. If you push the concentration of either too high, CaCO3 will start to precipitate. Magnesium interferes with this process, permitting both calcium and carbonate to be elevated above where they would be in the absence of magnesium.

If this sounds confusing, don't feel alone. In Stephen Spotte's book _ Captive Seawater Fishes, Spotte says "The study of carbonate minerals involves nuances of solubility that pose some of the most difficult problems in chemical oceanography and geochemistry."25 Nevertheless, the following section will attempt to give a simplified version that suits our level of understanding as aquarists.

How does magnesium interfere with precipitation of CaCO3? The primary way involves magnesium poisoning the surface of growing CaCO3 crystals, slowing the precipitation. It can, in fact, be slowed to the point where it simply does not happen at rates problematic to an aquarist. In the following discussion it is important to remember that, other things being equal, alkalinity is a good indicator of the concentration of carbonate. So higher alkalinity equates to higher carbonate.

In short, while magnesium carbonate is not supersaturated in seawater (or in typical reef aquaria), and will not precipitate on its own, magnesium is attracted to calcium carbonate surfaces where the carbonate ions are already held in place by the calcium ions. With the carbonate ions held in place, magnesium finds this an attractive place to bind.

After a short time in seawater, a virgin calcium carbonate surface quickly attains a thin coating of Mg/CaCO3 (magnesian calcite) as magnesium pushes its way into and onto the crystal surface. Eventually, the surface contains a substantial amount of magnesium. The extent to which this happens depends on the underlying mineral, and is apparently much more extensive on calcite than aragonite. It also depends upon the relative amounts of calcium and magnesium in the water. Regardless, a new type of material is formed that contains both calcium and magnesium.

This new mineral surface containing both calcium and magnesium is not a good nucleating site for precipitation of additional calcium carbonate (as aragonite or calcite), and precipitation of additional CaCO3 slows down substantially.

In Captive Seawater Fishes there is an extensive discussion of the impact of magnesium on the calcium/carbonate system, including a set of data that indicates the magnitude of the impact that magnesium can have.25 In this experiment, batches of artificial seawater were made up with varying magnesium and carbonate levels. The scientists then measured how long it took for calcium carbonate to precipitate from each solution. Not surprisingly, the higher the carbonate was raised, the more rapid was the precipitation of calcium carbonate.

More interestingly, the magnesium levels were found to have a very large impact on the rate of precipitation. In batches with no magnesium, and at natural calcium and elevated carbonate levels, calcium carbonate was found to precipitate in minutes. With a natural seawater level of magnesium added to that mix, the precipitation was delayed to 13 to 20 hours. With double the natural magnesium concentration, the precipitation was delayed to 22 to 29 hours.

Even more strikingly, at a lower level of carbonate (closer to that of natural seawater and probably similar to that in many reef aquaria), precipitation was delayed from a few minutes in the absence of magnesium to 750 hours in the presence of natural levels of magnesium. Consequently, magnesium has a big impact on the rate of precipitation of calcium carbonate (a fact that has been confirmed by many researchers).

But what does that have to do with a reef aquarium? One situation in which calcium carbonate can precipitate involves adding calcium carbonate seed crystals of some type to the aquarium. For example, by adding calcium carbonate sand or one of the calcium carbonate supplements like Aragamight or Kent's Liquid Reactor.26

A second situation where solid CaCO3 forms is when abiotic precipitation initiates in the aquarium.24 This precipitation happens when supersaturation is pushed to unusually high levels (either in the tank as a whole, or in localized regions). This rise in supersaturation can be caused by a rise in pH (which increases the amount of carbonate present by converting bicarbonate into carbonate), a rise in temperature (as on a heater or pump impeller; the temperature rise decreases the solubility of calcium carbonate and also converts bicarbonate into carbonate), or more directly by a rise in either calcium or carbonate.24

After the solid calcium carbonate has appeared in the system by whatever means, precipitation of CaCO3 will begin immediately. What processes inhibit continued precipitation of CaCO3 onto a growing crystal? The main thing happening in normal seawater is likely the impact of magnesium (though phosphate and organics may play an important role in some aquaria).24 This is the point that magnesium gets onto the growing surface of the crystal, essentially poisoning it for further precipitation of calcium carbonate. Since magnesium can reduce the likelihood or extent of calcium carbonate precipitation in this fashion, it thus acts to make it easier to maintain high levels of calcium and alkalinity.

Conclusions

Magnesium is an important ion for reef aquarists. In addition to its many biological functions, it serves to prevent the excessive precipitation of calcium carbonate from both seawater and aquarium water. Since both calcium and alkalinity are very important to organisms that we keep, making sure that they are not lost to excessive precipitation is an important part of aquarium husbandry.

Happy Reefing!

References

  1. Low-magnesium calcite produced by coralline algae in seawater of Late Cretaceous composition. Stanley, Steven M.; Ries, Justin B.; Hardie, Lawrence A. Morton K. Blaustein Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD, USA. Proceedings of the National Academy of Sciences of the United States of America (2002), 99(24), 15323-15326.
  2. New data on the relation between the magnesium content of reef-forming corals and their systematic location and stages of growth. Pozdnyakova, L. A.; Krasnov, E. V. Inst. Biol. Morya, Vladivostok, USSR. Doklady Akademii Nauk SSSR (1981), 260(3), 739-40 [Paleontol.].
  3. Magnesium content of calcite in carapaces of benthic marine Ostracoda. Cadot, H. Meade, Jr.; Kaesler, Roger L. Harris Cent. Conserv. Educ., Hancock, NH, USA. University of Kansas Paleontological Contributions, Papers (1977), 87 23 pp.
  4. Assessment of calcareous alga Corallina pilulifera as elemental provider. Yan, Xiaojun. Institute of Oceanology, Chinese Academy of Sciences, Tsingtao, Peop. Rep. China. Biomass and Bioenergy (1999), 16(5), 357-360.
  5. Calcium and magnesium carbonate concentrations in different growth regions of gorgonians. Velimirov, B.; Boehm, E. L. Dep. Zool., Univ. Cape Town, Rondebosch, S. Afr. Marine Biology (Berlin, Germany) (1976), 35(3), 269-75.
  6. Biochemistry of the coral. IX. Inorganic composition of the skeleton of the coral. Hosoi, Keizo. Sendai, Japan. Science Repts. Tohoku Univ. (1947), 18 85-7.
  7. Oxygen and carbon isotopic composition of carbonates deposited by red algae in the middle Adriatic. Dolenec, Tadej; Herlec, Uros; Pezdic, Joze. Fakulteta za Naravoslovje in Tehnologijo, Univerza v Ljubljani, Ljubljana, Slovenia. Rudarsko-Metalurski Zbornik (1995), Volume Date 1994, 41(3-4), 193-202.
  8. The marine bacteria. II. The specificity of mineral requirements of marine bacteria. Hidaka, Tomio. Univ. Kagoshima, Japan. Kagoshima Daigaku Suisangakubu Kiyo (1965), 14 127-80.
  9. Nutrition and metabolism of marine bacteria. II. The relation of sea water to the growth of marine bacteria. MacLeod, Robert A.; Onofrey, E. Pacific Fisheries Exptl. Sta., Vancouver, BC, Can. Journal of Bacteriology (1956), 71 661-7.
  10. Nutrition and metabolism of marine bacteria. I. Survey of nutritional requirements. MacLeod, Robert A.; Onofrey, Eva; Norris, Margaret E. Pacific Fisheries Expt. Sta., Vancouver, BC, Journal of Bacteriology (1954), 68 680-6.
  11. Epithelial transport of magnesium in the kidney of fish. Beyenbach K W; Freire C A; Kinne R K; Kinne-Saffran E Section of Physiology, VRT 826, Cornell University, Ithaca, N.Y. 14853 MINERAL AND ELECTROLYTE METABOLISM (1993), 19(4-5), 241-9.
  12. Magnesium -- Part II by Craig Bingman Aquarium Frontiers April 1999. http://www.animalnetwork.com/fish2/aqfm/1999/apr/bio/default.asp
  13. Persistent relaxation of the adductor muscle of oyster Crassostrea gigas induced by magnesium ion. Namba, Kenji; Kobayashi, Makoto; Aida, Satoshi; Uematsu, Kazumasa; Yoshida, Masayuki; Kondo, Yukie; Miyata, Yuji. Fac. Applied Biol. Sci., Hiroshima Univ., Hiroshima, Japan. Fisheries Science (1995), 61(2), 241-4.
  14. Chemical aid for shucking the Pacific oyster, Crassostrea gigas. Whyte, J. N. C.; Carswell, B. L. Dep. Fish. Oceans, Fish. Res. Branch, Vancouver, BC, Can. Canadian Technical Report of Fisheries and Aquatic Sciences (1983), 1238 33 pp.
  15. The Composition Of Several Synthetic Seawater Mixes by Marlin Atkinson and Craig Bingman Aquarium Frontiers March 1999. http://www.animalnetwork.com/fish2/aqfm/1999/mar/features/1/default.asp
  16. Reef Aquaria with Low Soluble Metals by Randy Holmes-Farley, Reefkeeping, April 2003. http://reefkeeping.com/issues/2003-04/rhf/feature/index.htm
  17. Calcium Carbonate for CaCO3/CO2 Reactors: More Than Meets the Eye by Craig Bingman Aquarium Frontiers, August 1997. http://www.animalnetwork.com/fish2/aqfm/1997/aug/bio/default.asp
  18. Alternative Calcium Reactor Substrates by Greg Hiller Aquarium Frontiers. http://www.animalnetwork.com/fish/library/articleview2.asp?Section=Aquarium%2BFrontiers%2B--%2BBiochemistry%2Bof%2BAquaria&RecordNo=1571
  19. Necessary Nutrition, Foods and Supplements, A Preliminary Investigation by Ronald L. Shimek. Aquarium Fish Magazine. 13: 42-53. http://www.animalnetwork.com/fish/data/foods.asp
  20. Magnesium Ion Precipitation in Reef Aquaria: A Tempest in a Teapot by Craig Bingman Aquarium Frontiers, July 1997. http://www.animalnetwork.com/fish2/aqfm/1997/jul/bio/default.asp
  21. A Homemade Magnesium Supplement by Craig Bingman Aquartium Frontiers, June 1999. http://www.animalnetwork.com/fish2/aqfm/1999/june/bio/default.asp
  22. Magnesium -- Part I by Craig Bingman, Aquarium Frontiers, March 1999. http://www.animalnetwork.com/fish2/aqfm/1999/mar/bio/default.asp
  23. Calcium and Alkalinity by Randy Holmes-Farley, Reefkeeping, April 2002 http://reefkeeping.com/issues/2002-04/rhf/feature/index.htm
  24. Calcium by Randy Holmes-Farley, Advanced Aquarist, March 2002. http://www.advancedaquarist.com/2002/3/chemistry
  25. Captive Seawater Fishes: Science and Technology by Stephen Spotte, Publisher: Interscience, 1992.
  26. Calcium Carbonate as a Supplement by Randy Holmes-Farley, Advanced Aquarist, July 2002. http://www.advancedaquarist.com/2002/7/chemistry
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