Reef chemistry as it happens!

What scientific reactions are taking place in our reef tanks? Levi Major unravels some chemical mysteries.

How does calcium carbonate form in my tank?
This is created in the aquarium by two main processes. Firstly, calcium and carbonate ions can be locked by living organisms, such as coral or coralline algae, within their calcium carbonate skeletons. Non-biological (abiotic) precipitation can also tend to occur.

For either to take place there’s a requirement for both calcium and carbonate ions to be in solution generally at or above the solution point.  

This can be taken as the point where the rate at which calcium and carbonate ions in solution land on a solid calcium carbonate surface and come out of solution is equalled by the rate at which they leave to become part of the solution.

If we raised the levels of calcium and/or carbonate within the solution it would be supersaturated — having more ions in solution than can be deemed stable over a long term.

Then precipitation of calcium carbonate can take place because the rate at which ions land on the surface of the solid exceeds the rate at which they can leave. Therefore, the solid surface is built up with further calcium and carbonate ions.

Even though more complex than abiotic precipitation, corals almost exclusively take calcium and carbonate ions from the solution to deposit their skeletons. The chemical composition of calcium carbonate (CaCO3-Ca2+ + CO3-2) indicates these organisms use a 1:1 ratio of calcium and alkalinity.  

However, the consumption of calcium can be seen to vary from species to species, due to the  incorporation of magnesium within calcium carbonate.

If we discover that we have very high calcium or alkalinity in our aquaria, precipitation of calcium carbonate can reduce the level of both in solution.

If we increase the levels of these ions so that they exceed the saturation point then the rate of calcium carbonate precipitation is increased. The result is accelerated coral growth.

Conversely, if the levels of calcium and/or carbonate ions in solution were below saturation point there would be no net gain in precipitation of calcium carbonate.

So should I just maintain high levels of calcium and alkalinity if I want fast coral growth?
Not exactly! The actual rate at which calcium carbonate is deposited can be affected by certain other water parameters, such as pH and magnesium.

Both bicarbonate and carbonate are forms of the same ion. At a lower pH, the bicarbonate form (HCO3-) predominates, whereas at a higher pH more of the carbonate form (CO32-) exists.  

The effect of varying pH can be acute, in that each drop of 0.3 pH units below a pH of 9 causes a two-fold drop in carbonate concentration.  

A full pH unit drop would correspond to a ten-fold decrease in carbonate concentration.

Accordingly, by varying the pH of a solution we can also change the amount of carbonate ion in solution. As mentioned, the concentration of these ions in solution determines the rate at which carbonate ions land on the surface of the solid.  So the higher the pH, the faster the rate at which these ions land. Therefore the solubility of calcium is lower at higher pHs.

Lower solubility implies that as pH increases the amount of calcium and alkalinity that can be kept in solution without precipitation occurring decreases.  

If we were to add kalkwasser (limewater) solution to the aquarium, pH would increase.  This would rapidly permit precipitation of calcium carbonate.

This is not necessarily as a result of increasing levels of calcium or alkalinity, but also due to the fact that as we increase pH much existing bicarbonate within the water converts to carbonate. Then there’s a resultant spike in carbonate concentration.

The opposite is true with a falling pH in that the amount of calcium and alkalinity that can be kept in solution without precipitation occurring increases. This is why adding carbon dioxide to a reactor dissolves the media.

You may think a lower pH is better as you can maintain calcium and alkalinity levels better and abiotic precipitation of calcium carbonate will not occur. However, your corals would have to work harder converting bicarbonate to carbonate to allow calcification.

How does the level of magnesium affect the levels of calcium and carbonate ions in solution? Does it affect the rate of calcium carbonate precipitation?

The role of magnesium is far more complex than pH and alkalinity.  If we examine standard seawater magnesium is the third most abundant ion after chloride and sodium, so its presence complicates our simplistic view of calcium and carbonate ions landing on and leaving the surface of our solid calcium carbonate.

Magnesium ions can be incorporated into the crystal structure of calcium carbonate and replace the role of the calcium ions.

As ever more magnesium is incorporated onto the surface of the calcium carbonate a layer of calcium and magnesium carbonate is formed. This results in the surface of the calcium carbonate no longer resembling one of calcium carbonate, preventing a firm bond of further calcium and carbonate ions. Further precipitation of calcium carbonate is largely reduced.

The extent to which magnesium is incorporated within calcium carbonate surfaces depends on the amount of ions in solution. The greater, the more it is incorporated.

If magnesium levels are lower than normal, it may not adequately get onto the growing calcium carbonate surfaces. This will then allow calcium carbonate to proceed even faster and may lead to increasing abiotic precipitation of the calcium carbonate.

Our inability to maintain adequate calcium and alkalinity levels, despite extensive supplementation or evidence of abiotic precipitation of calcium carbonate on heaters and pumps, means that the levels of magnesium are inadequate within the home aquarium.

Should I use supplements if my parameters seem fine?
Routine water tests will show your parameters and whether you need to supplement your tank beyond a water change. If you know what parameter is out of kilter, you should now know the best way to tinker and tune your set-up.

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