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Salifert Test – Magnesium


Salifert Test – Magnesium is for testing the magnesium level of saltwater aquariums. It is important that the magnesium level is right because it helps fish and invertebrates (especially corals) to grow in a healthy way.

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Salifert Test – Magnesium is for testing the magnesium level of your aquarium water.

This Salifert Profi Test kit is for testing saltwater (not freshwater).

It is important that the magnesium level is right for your animals. Magnesium helps fish and invertebrates (especially corals) to grow in a healthy way.

What is the right magnesium level for coral growth and coral colour?

Natural seawater has a calcium level of around 1280ppm. In an aquarium, you should always aim to keep your calcium level above 1220ppm.

As a rule of thumb your magnesium level should be roughly three times your calcium level. So if you were going to keep your calcium at 450ppm, we would say that your magnesium level should be 1350ppm. Those parameters are what we at Cornwall Aquatics would aim for in most reef aquariums. However, magnesium can be kept between 1220 – 1390ppm. The level that you choose will depend on what type of reef you have and whether you are aiming for more coral growth or more coral colouration or a happy medium between the two.

Foundation elements

The foundation elements are the essential building blocks of coral skeleton. If they are not available in the correct ratios, one of them will quickly become a limiting factor. Over 90% of coral skeleton is comprised of calcium carbonate, which is made by combining calcium and carbonate ions from the water. This combination can form a strong crystal structure known as aragonite or a more brittle crystal structure known as calcite. In unbalanced conditions (eg. where there are relatively low levels of magnesium) the skeleton will develop with a higher proportion of calcite. This would make a coral more brittle and more susceptible to damage.

Regular testing

It is important to keep the right balance between the foundation elements; KH, calcium and magnesium. This is especially important to achieve healthy coral growth. We recommend checking these three parameters at least once a week and buffering with additives if necessary. It is easy to test once you get the hang of it. If you need any help or advice our service team can book you in for an  Aquarium Health Check.

Magnesium in salt

Most salts are now designed to be used with RO or RODI water. This means that they have a good magnesium content. If you have been making regular water changes with a good quality salt and your magnesium is still too low, it could mean that your animals are using the magnesium from the water to grow. In this case you will need to add some magnesium buffer to slowly bring the magnesium of the aquarium up to the desired level.

The Salifert Magnesium Profi Test contains enough reagent to test 50 times.

Top tips:

Keep a record of you water parameter results and note what type and how much additive you use. This way you can see patterns over time to help you better maintain your aquarium.

If you need to add magnesium to your saltwater aquarium, it is first worth checking that your salinity is correct. If your salt level is too low, you may see that the magnesium is also low. And if the salinity is too high, you may see a high magnesium reading. Get your salinity right before you change the magnesium level with buffers.


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