Are all Kalkwasser the same?

[Origami]

I feel Mg is a very important aspect of Kalkwasser, even if it is not in the mixture.

 

Tom,

 

I think your right about my Mg consumption. But I also know I'm going thru Ca probably in the same speed. Since there both consumed by the flesh of the corals now for Strontium I think it is used in the calcium absorption process. But I'm not really sure all I know is there are trace amounts of Strontium in the tank.

 

Magnesium is certainly very important, I think, both for the role it plays in keeping our calcium and alkalinity in supersaturation and in the formation of coral skeletons. The rate of magnesium consumption, however, varies relative to calcium consumption because different corals apparently use different amounts of magnesium to make their skeletons. It's this, and other processes, that can often lead to long-term imbalance between alkalinity and calcium despite the use of balanced additives (such as kalkwasser), thus requiring the occasional re-balancing using unbalanced additives such as baking soda for alkalinity, magnesium chloride/sulfate for boosting magnesium, and calcium chloride for leveling calcium. But, because different corals consume different amounts of magnesium, the rate of consumption varies from tank to tank.

 

Now, as I look at it, your magnesium level is down around 140 ppm less than where your change water should be mixing up. Given that magnesium should only make up around 1% of your coral skeletons by weight (on average - see http://reefkeeping.com/issues/2006-07/rhf/index.php#6), or 2.5% by weight relative to calcium, it's difficult to fathom that magnesium has been added to the system at the same rate as calcium consumption. You just don't have enough calcium in the water to do this. Far more likely is that magnesium has not been replenished at the same rate as calcium replenishment. As it so happens, you're magnesium levels are consistent with levels found in NSW, so you're good. But, if your consumption is exceeding replenishment, you can expect that these levels will continue to fall over time and that you're going to have to supplement with an unbalanced additive such as Mag flake (magnesium chloride). Of course, your use of RC in your change water, which is high in magnesium, will help offset this imbalance.

 

Regarding magnesium, calcium, and strontium - if you were to look at the periodic chart of the elements and notice that they occupy the same column (Group 2) on the chart - are in the alkaline metals group. In many ways, they share similar properties including forming compounds similarly. That's why we see these elements being used in similar fashion in skeletal formation and it's the reason why magnesium is able to interfere with calcium precipitation in the supersaturated environment of our tank water. Strontium, in part, is used the same way.

 

That strontium may also play a part in calcium deposition (other than its use as a calcium analogue) has been proposed by some research (referenced here: http://www.advancedaquarist.com/issues/nov2003/chem.htm). There are also hobbyist reports of increased coral growth rates with the dosing of strontium, however no conclusive relationship from which we can infer the optimal strontium levels seems to exist yet. It's clear, however, that strontium in excess is toxic to many life forms.

 

I took a passing interest in strontium recently, going so far as to buy a bottle of Brightwell's Strontion (Strontium Chloride solution) and a (rather pricey) Salifert test kit. My going-in assumption was that I might have been low on strontium after having run my tank for nearly two years and not having ever dosed the stuff. What I found, however, was that my strontium levels were spot-on in the recommended range, presumably having been replenished through regular water changes. Unwilling to dose my tank to the maximum level of strontium in the recommended range (according to Salifert), the bottle still sits on the shelf, unused. Lesson learned.