How to Lower Magnesium in Reef Tank – 2 Effective Methods

Magnesium is one of the important factors that corals in the reef tank need. As the level of magnesium in ocean water is high, the presence of magnesium in the aquarium gives the coral a home-like state. But sudden increases in magnesium can be dangerous for some other aquatic animals. Thus it is necessary to know how to lower magnesium in reef tanks.

You can get the level down in a natural way and a chemical. In the natural process, you change 25% of the tank water. Do it once a week until you get the desired level.

In the chemical method, you use a chemical reactor. It has calcium hydroxide which reacts with magnesium and forms particles that can be filtered out. This method has some side effects that will be described in the process of using it later in the post.

In this post, you will get all the processes described step by step. Also, you will get to know all the side effects and consequences of ways of lowering the level of magnesium.

2 Ways To Lower The Level Of Magnesium In Your Reef Tank

The level of magnesium is a tricky business when it comes to balancing the level of magnesium. There are basically two approaches to bringing down the level of magnesium in the tank. One is a natural way and the other is the chemical approach. 

The natural way is more convenient as it doesn’t have many side effects. The chemical approach is not recommended unless it is of utmost necessity. Both of the approaches are described in detail below.

1. Natural Way: Water Change

check the level of magnesium in reef tank

Change of water is one of the most effective ways to balance the level of magnesium in the aquarium. When you introduce new water to the tank it creates the balance you need. 

But you can’t just randomly throw it away from the tank. The required steps are described below.

Step 1: Check The Level

First, you will need to check the level of magnesium. For this, you will find several kits in the market.

Once you have the kit measure the level of magnesium. It is healthy to have 1400 ppm. But if the level is 1500+ ppm then you will need to take further steps. It is wise to take steps to balance the level of magnesium if the kit shows 1460 ppm. 

Step 2: Change The Water

After determining the level of water it is time to change the water. There are two points you will need to consider. 

The first one is that you cannot change more than 25% water at a time. This will imbalance the other components of water. And the aquatic lives in the aquarium will have to restart the adaptation process. Every two weeks change 25% of the tank water. And continue doing it until the level is balanced.

The other point is checking the level of magnesium in the new water. Before you add 25% water check the level of magnesium in the water. It is best if the level is 0 ppm. This will balance the level more easily. 

You can also use distilled water. As they are purified to balance chemicals in almost perfect amounts. 

Step 3: Keeping Tracks

Just changing the water doesn’t mean the level of magnesium will go down. You will still need to keep track. Check the level every other day. 

Several factors can increase the level of magnesium. Thus before introducing any external food or fertilizer in the meantime, check the components. If they have magnesium in them then they need to be introduced cautiously. 

Repeat steps 1, 2, and 3 until the level of magnesium gets lower than 1400 ppm or the desired amount. With these procedures, you might need a longer time to adjust the level but it will be worth the wait. 

2. Chemical Removal: Using Calcium Hydroxide

Reef Tank Magnesium Level Testing

Not every time there is time to take the natural approach of water change to balance the level of magnesium. The level of magnesium is sensitive especially when the aquarium has varieties of coral. They can be sensitive to higher levels of magnesium, so you will have to take fast action.

In the chemical removal process, calcium hydroxide is used to accumulate magnesium into particles. Before the process, you also need to have a kit to test calcium and alkalinity. 

It only has five steps.

Step 1: Check The Level Of Magnesium

Before putting the water through the chemical reactor, it is important to test the level of magnesium. If the level of magnesium is above 1400 ppm only then use the chemical removal process.

If the level of magnesium is about 1000 ppm and uses a chemical removal process, then there’s a chance that the level will fall very low and create a deficiency. 

Step 2: Check The Level Of Calcium And Alkalinity

During this process the level of calcium increases in the water. As a result, if any aquatic animal is sensitive to the level of calcium it can be harmed by the level. 

If the level is above 500 ppm then the procedure can be risky. Also, make sure not to use any calcium supplements before the procedure. This will increase the level of calcium to a dangerous scale.  

The same goes for the level of alkalinity. In this process the calcium and the alkalinity of the water increase. You need to ensure that the level of alkalinity is between 7 to 10 dKH. 

Step 3: Run The Water Through The Reactor

After testing the level of magnesium, calcium, and alkalinity it is time to run the water through the reactor. In the reactor, there will be calcium hydroxide. When the water passes through the reactor the magnesium in the water does a chemical reaction. 

In the chemical reaction, the calcium is replaced by magnesium and produces magnesium hydroxide. They form particle-like bonds and can be removed by filtering. But in this process, the calcium will be left in the water. Thus it is said that the process will increase the level of calcium in the water. 

Step 4: Filter Out The Magnesium Hydroxide

Once the reactor is done with its reaction, it is time to filter out the particular form of magnesium. For this, you will need to have a filter sock. They must have fine pores so that the magnesium does not get passed.  

You can fit the sock on the filter that is already installed in the aquarium. Also can use it manually to collect the particle. The first process is much easier than doing it manually. It also ensures that all the particles will be taken out properly.

Step 5: Checking The Level Further

Once you are done with the filtration it is again necessary that you check the level of magnesium, calcium, and alkalinity. Use new kits and determine the level. 

The magnesium level should not be higher than 1400 ppm or less. The level of calcium should be between 400 ppm to 450 ppm and in the case of alkalinity, it should be between 7 to 10 dKH. 

If the level of magnesium is higher than you desire then take steps 1 to step 4 again.  

Importance Of Magnesium For Reef Tanks

Even though here we are talking about lowering the level of magnesium, it is necessary to know that magnesium is a very important factor in the growth of coral. At 1350 to 1400 ppm of magnesium, corals thrive in development.

Also, it balances the level of calcium and alkalinity in the reef tank. When the level of magnesium goes down it disrupts the presence of calcium and alkalinity. They will fall out in the filter and ultimately the level will drop.

In a reef tank, we try to replicate the natural state of the ocean. This is very necessary for corals to survive, as they are very sensitive animals. In the ocean, the consistency of magnesium is very high. They stay in the form of salt and help in the growth of the corals. 

Besides calcium, magnesium is another element that forms the skeleton of corals. Thus when there is a deficiency of magnesium or lower levels it starts to decay the coral.  

Why Reduce The Level of Magnesium

Even though the presence of magnesium is very important to the corals it doesn’t do much favor to the invertebrate animals. When there is a sudden hike in the level of magnesium it affects the life of invertebrates in the aquarium. 

On the other hand, both fishes and corals love the presence of magnesium. But they can also tolerate the level maximum of 2000 ppm. You need to start taking action when the level is already 1500 ppm. 

Sometimes the level can reduce on its own due to filtration and other factors. But when it’s too high at least do the water change procedure to avoid any harm to the aquatic life. Especially the invertebrates. 

Invertebrate animals don’t have many bones in them. Thus excess magnesium doesn’t get absorbed by the body of the invertebrates properly. When they consume excess magnesium, it creates toxic chemicals. This leads to the death of the animals. 

How To Avoid Sudden Rise Of Magnesium Level

A sudden change of any chemical formation in the aquarium can cause stress to any form of aquatic animal in the reef tank. Thus here are some pointers that can help you to stop the sudden change of magnesium.

  • Algae wafers contain magnesium in them. Thus before adding them to the tank check the level of magnesium in the tank.
  • Before adding any fertilizer or spot feeding any coral check the ingredients of the products.
  • If the level of calcium is low, check the level of magnesium. 
  • Try not to put invertebrate animals along with the corals. Thus the level of magnesium won’t have much effect on aquatic animals in the reef tank.


How fast does magnesium deplete in reef tanks?

35 ppm magnesium drop each day can not be anything other than a test blunder. Genuine magnesium utilization is more similar to 0-2 ppm each day, and can’t be a lot higher. It truly can’t be considerably more than 1/tenth of the calcium drop rate.

How high is too high for magnesium in a reef tank?

Corals love the entirety of that magnesium and they will flourish in it. The issue is, that degrees of magnesium that surpass 1350 is not something friendly to anything else. Magnesium levels that surpass 2000 ppm will kill your invertebrate creatures going in a rush.

Is magnesium good for fish?

Both magnesium and calcium are supplements for sea-going plants. Magnesium, very much like the significant plant supplement iron, is answerable for the arrangement of chlorophyll. Aquarium plants just consume little magnesium, yet they ought to be accessible in adequate amounts (north of 10 mg/l is suggested).

Is magnesium in seawater?

Magnesium is available in seawater in measures of around 1300 ppm. After sodium, it is the most usually tracked down cation in seas. Streams contain roughly 4 ppm of magnesium, marine green growth 6000-20,000 ppm, and clams 1200 ppm. Dutch drinking water contains somewhere in the range of 1 and 5 mg of magnesium for each liter.

When should I test my magnesium levels?

Magnesium testing might be requested as the development of low blood levels of calcium and potassium persists. It likewise might be requested when an individual has side effects that might be because of a lack of magnesium, like muscle shortcoming, jerking, squeezing, disarray, cardiovascular arrhythmias, and seizures.

Final Words

Magnesium is a necessary and important element for corals in the reef tank. But excess of any elements, even if they are necessary, is not good. Magnesium causes deadly effects on the invertebrates in aquariums. 

Therefore keep a check on the level of magnesium if you have invertebrates in your reef tanks. If the level is over 1500 ppm take the necessary steps. This describes to you the whole process of how to lower magnesium in reef tanks in brief.

Howard Parker

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