High Magnesium in Reef Tank, How to Lower & Maintaining Proper Levels?

Magnesium is something every reef tank needs, and it is important to achieve the proper levels, or you could end up experiencing problems with your plants and aquatic wildlife. Low levels of magnesium tend to be the culprit in these cases, but high levels should also be monitored.

Hobbyists usually strive to maintain magnesium levels similar to those found in the wild, in the ocean, and you may need to take action if the magnesium levels in your tank won’t stabilize on their own.

In this article, we are going to cover what magnesium is, its benefits in the tank and what you can do if your levels are too high or too low.

Magnesium & Reef Tanks

A healthy reef tank needs appropriate levels of magnesium, and it is essential to test the water before you start experimenting with additives. Saltwater tanks will usually have a magnesium level of between 1250 and 1350 ppm, and as all tanks are different, some tanks will be closer to 1250 while others test closer to 1350.

To maintain the magnesium levels in your tank, it is recommended to make it a habit to test the water at least once every other week. An interesting fact is this: water hardness is measured by looking at the concentration of magnesium ions and calcium.

It is not uncommon to have problems with plants and animals in a tank, only to discover that the magnesium levels are either too high or too low, and you may need to dose regularly if you discover the magnesium levels to be on the lower side.

High Levels of Magnesium

If your magnesium levels are above 1350 ppm, this is something you might want to correct even though high levels of magnesium aren’t necessarily known for causing problems. 

Low levels of magnesium, yes, but high levels are harder to achieve and maintain, and do not tend to present an issue even if it happens. In a case where you do need to lower the levels, you will usually need to start by changing the water.

How to lower magnesium in reef tanks?

Lowering magnesium in reef tanks is something tank owners may ask themselves when seeing magnesium levels of 1400+ ppm, but there is seldom a reason to worry. Even levels as high as 1500 ppm are usually harmless, but if you do wish to lower it, a water change is your best bet (remember to use water with a lower magnesium salt mix).

Low Levels of Magnesium

A low level of magnesium can be caused by many things. One of the most obvious explanations is that the water isn’t changed often enough so that the existing magnesium gets used up and consumed without being replenished fast enough.

Another contributor to low levels of magnesium is if you are using the wrong type of salt for your reef tank. The salt in your tank should be one designed to be used in a saltwater reef aquarium, or else it might not have the levels of magnesium required for your organisms to thrive.

Signs of Low Magnesium Levels

Clear signs of a tank that is low in magnesium are when you are starting to have a hard time stabilizing the levels of alkalinity and calcium. If you continue to add additives, yet when you check your levels the next day it still isn’t right, then you might need to adjust the magnesium levels in the water.

If you are noticing polyp stony coral tissue loss, or bleaching of your small polyp stony corals or your coralline algae, these are also tell-tale signs of low magnesium levels in your tank. 

Magnesium & Its Effect on Corals

How about corals? Do corals need magnesium? The quick answer is yes, they do. For your tank corals, magnesium is essential for their metabolic function, and it enables corals to grow strong and to stay healthy long-term. 

Proper calcium and alkalinity still tend to be considered more important than magnesium levels in a tank with corals, but what some tank owners may not be aware of is that magnesium actually helps maintain higher levels of alkalinity and calcium in the water, without having to add supplements. 

Is Fish Affected by Magnesium?

All aquatic animals and plants need magnesium and calcium, and it is important to maintain the magnesium levels close to what saltwater fish is used to in the wild. Many species are very delicate and can be greatly affected if the magnesium levels in the tank are off.

Dosing Magnesium to Equilibrate Levels

The first step is to test your tank water to see what you are dealing with, and this is recommended even if you just tested the water the week before as it can fluctuate. High levels aren’t a cause for concern unless you are experiencing problems in your tank, but if your levels are low, you might need to start dosing.

Next, you need to choose a product that suits your needs, depending on the current state of your tank and how much more magnesium it needs. No two saltwater tanks are exactly the same, and it may take some time to figure out what works for your tank.

Some tanks have a higher use for magnesium, depending on what you have in your tank and your overall setup, and in these cases, dosing with a quality product might be required more often.


There is a lot to learn about the magnesium in your tank, but the most important thing is to try and keep your magnesium levels between 1250 and 1350 ppm. Somewhat higher levels do not tend to cause issues, but low magnesium levels could cause coral bleaching and tissue loss, among other things.

The easiest way to regulate the magnesium levels in your reef tank is by simply changing the water, and if the levels are low you might need to dose with a suitable product.

Check your water regularly to stay on top of what is happening with your tank’s magnesium levels, and remember that depending on your setup – your tank may use up the magnesium in the water faster than you are replacing it.


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