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Old 23-09-2007, 10:15 PM   #3
valerossi
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Magnesium

Magnesium's primary importance is its interaction with the calcium and alkalinity balance in reef aquaria. Seawater and reef aquarium water are always supersaturated with calcium carbonate. That is, the solution's calcium and carbonate levels exceed the amount that the water can hold at equilibrium. How can that be? Magnesium is a big part of the answer. Whenever calcium carbonate begins to precipitate, magnesium binds to the growing surface of the calcium carbonate crystals. The magnesium effectively clogs the crystals' surface so that they no longer look like calcium carbonate, making them unable to attract more calcium and carbonate, so the precipitation stops. Without the magnesium, the abiotic (nonbiological) precipitation of calcium carbonate would likely increase enough to prohibit the maintenance of calcium and alkalinity at natural levels.

For this reason, I suggest targeting the natural seawater concentration of magnesium: ~1285 ppm. For practical purposes, 1250-1350 ppm is fine, and levels slightly outside that range (1200-1400 ppm) are also likely acceptable. I would not suggest raising magnesium by more than 100 ppm per day, in case the magnesium supplement contains impurities. If you need to raise it by several hundred ppm, spreading the addition over several days will allow you to more accurately reach the target concentration, and might possibly allow the aquarium to handle any impurities that the supplement contains.

An aquarium's corals and coralline algae can deplete magnesium by incorporating it into their growing calcium carbonate skeletons. Many methods of supplementing calcium and alkalinity may not deliver enough magnesium to maintain it at a normal level. Settled limewater (kalkwasser), in particular, is quite deficient in magnesium. Consequently, magnesium should be measured occasionally, particularly if the aquarium's calcium and alkalinity levels seem difficult to maintain. Aquaria with excessive abiotic precipitation of calcium carbonate on objects such as heaters and pumps might suffer from low magnesium levels (along with high pH, calcium, and alkalinity).

Phosphate

The "simplest" form of phosphorus in reef aquaria is inorganic orthophosphate (H3PO4, H2PO4-, HPO4--, and PO4--- are all forms of orthophosphate). Orthophosphate is the form of phosphorus that most test kits measure. It is also present in natural seawater, although other forms do exist there as well. Its concentration in seawater varies greatly from place to place, and also with depth and with the time of day. Surface waters are greatly depleted in phosphate relative to deeper waters, due to biological activities in the surface waters that sequester phosphate in organisms. Typical ocean surface phosphate concentrations are very low by reefkeeping standards, sometimes as low as 0.005 ppm.

Absent of specific efforts to minimize the phosphate level, it will typically accumulate and rise in reef aquaria. It is introduced mostly with foods, but can also enter with top-off water and in some methods of calcium and alkalinity supplementation.

If allowed to rise above natural levels, phosphate can cause two undesirable results. One is inhibition of calcification. That is, it can reduce the rate at which corals and coralline algae can build calcium carbonate skeletons, potentially stunting their growth.

Phosphate can also be a limiting nutrient for algae growth. If phosphate is allowed to accumulate, algae growth may become problematic. At concentrations below about 0.03 ppm, the growth rate of many species of phytoplankton depends on the phosphate concentration (assuming that something else is not limiting growth, such as nitrogen or iron). Above this level, the growth rate of many of the ocean's organisms is independent of phosphate concentration (although this relationship is more complicated in a reef aquarium containing iron and/or nitrogen sources such as nitrate above natural levels). So deterring algae growth by controlling phosphate requires keeping phosphate levels quite low.

For these reasons, phosphate should be kept below 0.03 ppm. Whether keeping it below 0.01 ppm will yield substantial additional benefits remains to be established, but that is a goal that some aquarists are pursuing with various ways of exporting phosphate. The best ways to maintain low levels of phosphate in normal aquaria are to incorporate some combination of phosphate export mechanisms, such as growing and harvesting macroalgae or other rapidly growing organisms, using foods without excessive phosphate, skimming, using limewater, and using phosphate binding media, especially those that are iron-based (which are always brown or black). Some aquarists have also tried to reduce phosphate by inducing blooms of microorganisms such as bacteria. This last method should, in my opinion, be left to experienced aquarists.

Ammonia

Ammonia (NH3) is excreted by all animals and some other aquarium inhabitants. Unfortunately, it is very toxic to all animals, although it is not toxic to certain other organisms, such as some species of macroalgae that readily consume it. Fish are not, however, the only animals that ammonia harms, and even some algae, such as the phytoplankton Nephroselmis pyriformis, are harmed by less than 0.1 ppm ammonia.15

In an established reef aquarium, the ammonia produced is usually taken up rapidly. Macroalgae use it to make proteins, DNA, and other biochemicals that contain nitrogen. Bacteria also take it up and convert it to nitrite, nitrate, and nitrogen gas (the famous "nitrogen cycle"). All of these compounds are much less toxic than ammonia (at least to fish), so the ammonia waste is rapidly "detoxified" under normal conditions.

Under some conditions, however, ammonia may be a concern. During the initial setup of a reef aquarium, or when new live rock or sand is added, an abundance of ammonia may be produced that the available mechanisms cannot detoxify quickly enough. In these circumstances, fish are at great risk. Ammonia levels as low as 0.2 ppm can be dangerous to fish.16 In such instances, the fish and invertebrates should be removed to cleaner water, or the aquarium treated with an ammonia-binding product such as Amquel.

Silica

Silica raises two issues. If diatoms are a problem in an established reef aquarium, they may indicate a substantial source of soluble silica, especially tap water. In that case, purifying the tap water will likely solve the problem. In such a situation, testing may not reveal elevated silica levels because the diatoms may use it as quickly as it enters the aquarium.

If diatoms are not a problem, then I suggest that many aquarists should consider dosing soluble silica. Why would I recommend dosing silica? Largely because creatures in our aquaria use it, the concentrations in many aquaria are below natural levels, and consequently the sponges, mollusks, and diatoms living in these aquaria may not be getting enough silica to thrive.

I suggest dosing sodium silicate solution, as it is a readily soluble form of silica. I dose a bulk grade of sodium silicate solution (water glass), which is very inexpensive. You may find "water glass' in stores because consumers use it for such activities as preserving eggs. Finding chemicals to buy can be difficult for many people, however, and this linked hobby chemistry store sells to individuals. Ten dollars plus shipping buys enough to last for 150 years of dosing a 100-gallon aquarium, so cost is not an issue.

Based on my dosing experience, aquarists are probably safe dosing to 1 ppm SiO2 once every 1-2 weeks. This is based on the fact that my aquarium uses that much in less than four days without any sort of "bad" reaction. Of course, there's nothing wrong with starting at a tenth of that dosage and gradually ramping it up. If you do get too many diatoms, just back off on the dosing. I presume that all of the SiO2 I have added to my aquarium has been used by various organisms (sponges, diatoms, etc), but perhaps I have more sponges than other aquarists. Consequently, diatoms may be more of a concern in some aquaria than in mine.

I would also advise occasionally measuring the soluble silica concentration in the water, in case the demand in your aquarium is substantially less than mine. If the concentration started to rise above 3 ppm SiO2, even in the absence of diatoms, I would probably reduce the dosing rate because that is close to the maximum concentration that surface seawater ever contains. Additional details on dosing amounts and methods are described in this previous article.
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