How to Lower Calcium and Magnesium in Reef Tank {When Does It Get Dangerous?}

Are you concerned about Calcium and Magnesium levels getting too high in your reef tank? What happens to your reef tank and marine life when these levels get too high?

In this article, we’ll discuss the topic of lowering your Calcium and Magnesium in a reef tank.

How to Lower Calcium and Magnesium in Reef Tank: Perform tests every two weeks to check calcium and magnesium readings. Perform water changes to lower magnesium levels. Stop dosing your tank with calcium until alkalinity increases to help keep the balance for healthy corals and marine life to thrive. 

How Do I Lower the Magnesium in My Reef Tank?

You will need to test and accept that your reef tank requires magnesium levels between 1250-1350 ppm. Test the water once every two weeks to find out where your magnesium levels are.

Before you look into additives or supplements, observe your tank for any abnormal changes to the quality of the reef tank and health of your marine life.

You may reach levels of 1500 ppm and notice no changes or harmful effects. Remember that if you want to reduce magnesium levels, you will need perform water changes.

Use low magnesium salt mixed with water to obtain optimal readings:

  • 1250-1350 ppm are healthy levels of magnesium. 
  • Water changes help you lower magnesium.
  • Magnesium test kits help give you accurate readings once every 2 weeks.

How Do I Lower Calcium in My Reef Tank?

If you want to reduce the calcium in your reef tank, you will have to temporarily stop adding in supplements for a short amount of time while you test your reef tank water.

  • The optimal range of calcium should be: 380-450 ppm

Once you reach 450 ppm, you can begin dosing equal parts of alkaline and calcium to maintain a healthy balance.

Can Calcium Be Too High in a Reef Tank?

Yes. If calcium levels rise higher than 500 ppm you will notice a drop in alkaline. When the alkaline imbalance is too high, calcium will drop.

Your calcium aim is 380-450 ppm with alkalinity at 2.5-3.5 meq/L (7-10 dKH).

Will Raising Magnesium Lower Calcium In A Reef Tank?

Magnesium is indirectly related to the balance of calcium and alkalinity. It’s crucial nonetheless in a reef tank or else the balance in a reef tank falls (precipitates).

The corals in a reef tank will not be able to absorb essential elements. Your reef tank will be off balance. Maintain the following amounts:

  • Magnesium: 1250-1350 ppm
  • Calcium: 380-450 ppm
  • Alkalinity: 2.5-3.5 meq/L (7-10 dKH)
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What Does Too Much Magnesium Do in a Reef Tank?

An overdose of magnesium could harm your marine life including the invertebrates at the substrate. Snails and clams could become inactive, lethargic and weakened. Most likely, they will get eaten by larger marine life.

Your fish may also experience:

  • erratic swimming
  • lethargy
  • stunted growth in fishes

Also watch your corals closely during this time. They may lose their vibrancy and become unhealthy as well. Perform water changes to lower magnesium levels.

What Does Too Much Calcium Do In A Reef Tank?

If your calcium levels are higher than 450-500 ppm, you will notice some abnormal appearances to your corals and reef life. Corals may be stunted and their growth will slow down.

The tank itself will either have too much or too little alkalinity as a result. The pH of your aquarium will also be thrown off. For the health of your coral, maintain levels of calcium at 380-450 ppm.

Do Soft Corals Need Magnesium?

Soft corals are a welcome addition to reef tanks. They have a higher demand for magnesium than most other types of coral. You will need to add magnesium supplements to maintain the health of your soft coral.

If your magnesium levels read too high, you can balance it out with water changes. Your optimal readings should be kept at 1250-1350 ppm.

How Do You Check Magnesium in a Reef Tank?

Visit your local fish shop or look online for various magnesium test kits. A one milliliter syringe or micro scoop should allow you to test the magnesium levels when you take a sample of your tank water out.

Each test kit is slightly different and requires you to follow the directions in their instruction booklets.

Look for a certificate of accuracy and quality assurance when purchasing a magnesium test kit. Some cheaper types are known to keep inaccurate readings.

How Do I Test My Aquarium for Calcium?

Pour out a sample of your reef tank water in a separate container. Mix in the solution contained in the calcium test kit. Add 10 drops or follow the directions in the instructions.

Do not place your finger in or over the end of the cap. This could alter your calcium readings.

How Do I Know If Magnesium Levels Are Low?

You should be testing your reef tank once every 2 weeks for magnesium levels. You may notice the following signs if your reef tank is low on magnesium:

  • polyp stony tissue loss
  • small polyp stony corals looking bleached
  • coralline algae growth

Your corals need magnesium for healthy growth and metabolic function. Add in magnesium appropriately and perform water changes if your magnesium levels are high.


Maintaining a reef tank comes with plenty of tests that need to be undertaken in order to keep a healthy balance. You are recreating a mini-ocean in your aquarium and you will need the right balance of magnesium, calcium and alkalinity to get you to an optimal level for your corals and marine life to thrive.

Stunting their growth or limiting their metabolic function are side effects of too much or too little of these essential elements.

Water changes help to balance and lower calcium or magnesium. Continue testing your tank every 2 weeks and also watch your corals closely to let them show you if all is well.


Thank you for stopping by at We have plenty of articles on a wide variety of marine life that may also pique your interest. See you again soon!

Brian Arial

Brian Arial has kept fish for leisure and worked with fish stores for most of his life. He enjoys writing and caring for aquariums and ponds.

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