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Magnesium Levels in Reef Aquariums

Introduction to Magnesium

Welcome back everyone, it’s time for more chemistry. This blog will focus on magnesium which is an often overlooked ion in our reef tanks.

Reef hobbyists interested mainly in stony corals tend to tunnel vision in on calcium and alkalinity. On one hand, that makes sense because coral skeletons are primarily made up of calcium carbonate. A hobbyist needs to make sure those two ions are bioavailable because corals take in calcium ions and carbonate ions to grow.

On the other hand, it’s an incomplete view of what’s happening in our water and, keeping those levels can be challenging despite our best efforts.

Here is where testing for Magnesium can help. So what is the big deal with magnesium? First off, it is the third most abundant ion in salt water. The two most abundant ions for those that are curious are sodium and chloride, which really should come as no surprise.

Magnesium is typically measured in parts per million. Parts per million however is a slightly deceptive sounding unit of measurement. It is a mass ratio rather than a count of ions in solution. For example, magnesium represents 1300ppm while calcium is around 425ppm.

Based on these numbers one would assume magnesium is 3x more abundant than calcium, but in reality it is a whole lot more. Parts per million takes into account the mass of the ions and it just so happens that Calcium is significantly heavier than magnesium. When you compensate for the difference in mass, magnesium is actually 5 times more abundant than calcium in salt water.

The sheer abundance of Magnesium is significant because like Calcium, Magnesium ions carry two positive charges in solution. As such the two ions behave similarly and are utilized by our coral inhabitants in like ways. When stony corals form their skeletons, magnesium is regularly substituted for calcium.

When reefers only are testing for calcium and alkalinity they are essentially blind to this third ion in saltwater doing a lot of work behind the scenes. Magnesium provides buffering capacity by bonding with carbonate ions. Because magnesium is binding up carbonate, it allows for more carbonate in the water than if magnesium wasn’t there. When magnesium levels are appropriately high, it allows for the addition of calcium without the associated drop in alkalinity.

Testing for Magnesium

So now that we’ve gone over what Magnesium is and why it’s important, we can discuss doing a Magnesium test made by Salifert (Amazon Affiliate Link: It’s the same brand as the ones I showed in the calcium and alkalinity blogs (but I promise I’ll show other types of test kits in the future!).

The test is a basic titration. A good titration will show an immediate change once the buffering capacity is exceeded. We want to see at what point the sample changes color from this pink color to a blue color. Taking a look at where the stopper ended up, it looks like it reads .15.

This wraps up the major ion holy trinity of calcium, alkalinity, and magnesium.


Thanks again for watching! If you enjoy this blog and would like to support it, please take a look at the following Amazon Affiliate Link for the Magnesium Test Kit shown in the video:

Happy reefing everyone!

Than Thein

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