Lowering Magnesium in Reef Tanks: A Comprehensive Guide

If you own a reef tank, you know how important it is to maintain the right balance of minerals for your fish and coral. One of the most critical elements for a healthy reef tank is magnesium. However, too much or too little magnesium can have serious consequences on your aquarium’s ecosystem. In this guide, we’ll explore how to lower the levels of magnesium in your reef tank and keep it at an optimal level.

What is Magnesium?

Magnesium is a cationic element that plays a vital role in many biological processes, including photosynthesis, metabolism, and nerve function. In reef tanks, magnesium is essential for the growth and survival of corals, as it supports their ability to build calcium carbonate structures. However, excess magnesium in the tank can lead to algae blooms, causing your aquarium to become murky and unsightly.

Causes of High Magnesium Levels

There are several reasons why you may have high levels of magnesium in your reef tank.

Some of the most common causes include:

  1. Overfeeding: Overfeeding can cause an increase in phosphate and nitrate levels, which can lead to an algae bloom in your tank. This can result in higher magnesium levels as well.
  2. Poor Water Changes: If you don’t change the water in your reef tank regularly, you may be adding more minerals to the water, including magnesium.
  3. Overuse of Fertilizers: Using too much fertilizer can lead to an overabundance of nutrients in the water, which can cause algae growth and an increase in magnesium levels.
  4. Inadequate Filtration: If your filtration system is not working properly, it may be unable to remove excess minerals from the water, including magnesium.

How to Lower Magnesium Levels

Now that we’ve identified some of the common causes of high magnesium levels in reef tanks let’s explore how to lower them.

  1. Reduce Phosphate and Nitrate Levels: The first step in lowering magnesium levels is to reduce phosphate and nitrate levels in your tank. This can be done by reducing the amount of food you feed your fish, performing more frequent water changes, and using a filtration system that removes excess nutrients from the water.
  2. Use Water Conditioners: Water conditioners are designed to remove excess minerals from the water before they enter your tank. They work by binding with the minerals in the water and preventing them from being absorbed into your aquarium’s ecosystem.
  3. Add Activated Carbon: Activated carbon is a natural filter that can help remove excess nutrients from the water, including magnesium. It works by adsorbing the nutrients onto its surface as the water passes through the filter.
  4. Change Your Filtration System: If your current filtration system is not able to remove excess minerals from the water, you may need to upgrade to a more efficient model. There are many different types of filters available on the market, so it’s important to choose one that will meet your specific needs.
  5. Perform Regular Water Changes: Finally, regular water changes are essential for maintaining the right balance of minerals in your reef tank. It’s recommended that you perform at least 20% of your tank volume in water changes every two weeks.


Q: How often should I test the magnesium levels in my reef tank?

A: You should test the magnesium levels in your reef tank at least once a week to ensure that it’s at an optimal level.