Magnesium: The magic that holds your reef tank together.


Did you know that magnesium is the element that essentially makes reefing possible? Magnesium is used by corals for metabolic function to stay healthy and grow. However, since it's not largely or quickly consumed, it's pretty easy to maintain with infrequent additions of a supplement like magnesium chloride, or even just water changes.

So, what's the magnesium doing in your tank? Let's back up a bit.

If you caught our last two videos, you know that the corals in your tank are pulling both calcium and alkalinity (which we'll refer to as carbonate) from the surrounding water to support its overall health and growth. The problem is that the calcium and carbonate ions also have quite the affinity for each other, and will join in the tank to form calcium carbonate crystals. You might see the results of this as a white buildup on on your heaters and pumps, or even tiny particles settling in your sand bed.

Here's where the magnesium comes in! The magnesium ions in the tank prevent the calcium carbonate crystals from growing by incorporating itself into the growing calcium carbonate crystal, which makes it unattractive to additional calcium and carbonate ions. This is what makes it possible to maintain appropriate levels of calcium and alkalinity. Low magnesium levels will likely cause you problems when trying to maintain proper calcium and alkalinity levels.

How can we raise and maintain our magnesium levels? You have two options: magnesium chloride and magnesium sulfate. Magnesium chloride is the most abundant ion in natural seawater, which makes it the most popular option for reefers because additions have minimal impact to overall water chemistry.

The first step to finding out how much magnesium your tank needs is testing your tank for current levels. Two of the most common test kits are made by Salifert and Red Sea. We like the Red Sea kit because it's high quality and has inexpensive refill kits, which makes it the lowest overall cost in the long run. Once you know your current levels, you can use the reef calculator to identify the amount you need to add to reach your desired levels. 1350ppm is an ideal level, however, anywhere from 1320ppm to 1380ppm is recommended.

You can select which product you want to use to raise your levels. We recommendation keeping a jug of magnesium solution premixed and on hand. Ideally, this is a mix of 7 3/4 cup magnesium chloride and 3/4 cup magnesium sulfate, with enough purified water to make a gallon of solution. If you don't have magnesium sulfate, most reefers find it acceptable to skip and just use 8 cups of magnesium chloride.

A few notes on mixing; use purified water like distilled or RO/DI, and shake until fully mixed. There's a lot of salt being mixed here, so this will take some shaking! If you see salt crystals at the bottom of the jug, let it sit overnight or float it in some hot water. It's worth noting that the BRS magnesium additives are pharmaceutical grade, so as long as you use purified water it should result in a crystal clear solution. Finally, it's pretty common for salt mixes to be low in magnesium, probably because its one of the more expensive components. You can add magnesium directly to your salt mixing container to give it a boost prior to a water change.