I'd try bumping it up with some calcium chloride.
Kev, this is that
Seachem reef advantage Calcium)
However, it is not straight CaCl, such as Kent's Turbo, which would be a better choice I think.
What should I do now? Is there a way to lower Magnesium? Do you think the Alk is a contributing factor?
Water Change with some IO
Usually a higher than normal Alk and Mg lets more Ca to be dissolved in water.
John
A good read for you;
Magnesium
http://www.advancedaquarist.com/issues/oct2003/chem.htm
from it:
"Effect of Magnesium on the Calcium/Alkalinity Balance in Aquaria
How does magnesium impact the balance of calcium and alkalinity 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. 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. 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.
A second situation where solid CaCO3 forms is when abiotic precipitation initiates in the aquarium. 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.
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). 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."
