How To Lower Magnesium In Reef Tank: Do’s And Don’ts

Maintaining a correct parameter ensures the good health of our aquarium species.

Magnesium is one of the most important parameters to keep in check. 

Some foods and algae wafers are high in magnesium. When they remain uneaten, they dissolve in the water and increase the magnesium level. 

How to lower magnesium in reef tanks?

Lowering magnesium level is the simplest parameter to control out of all others. Simple water changing is sufficient most of the time if we do it right.

But if water changing doesn’t help, there are other small tricks. We could filter it with chemicals such as calcium hydroxide. But this method imposes some side effects. 

Wanna know more? Read this whole thing through, and you’ll be good to go!

What Is The Correct Level of Magnesium in a Reef Tank?

It is seldom that we need to worry about high magnesium levels. This is because sea waters are rich in magnesium. Because of this, aquatic species are used to high levels of magnesium. 

A more common problem has low magnesium levels. Fishes and corals are more affected by this factor.

When this happens, we have to go for magnesium supplements to increase the level in the reef tank. 

Until the level is within 2000ppm, we’re safe. Hence, we do not need to stress too much about lowering magnesium levels until then. 

Now, usually, it is advised to keep the magnesium within 1500. So, why is it that we don’t have to stress unless it goes beyond 2000?

When the level is below 2000, we just have to give our aquarium some time. This is because, with time, the magnesium goes down within a day or two.

And aquatic species can endure this level of magnesium for a short time.

The magnesium level will not exceed 2000ppm unless we add something extra to the tank. 

When it does go beyond that level, we need to start worrying.

Solutions To Reduce Magnesium Level

Magnesium is one of the most important water parameters to keep in check. If our magnesium level is too high, don’t worry! We got three solutions for you on how to get the perfect level of magnesium. 

Get The Correct Type Of Test Kit

This is the first step. Many times, we don’t get the correct kind of test kit. This leads us to collect incorrect results on magnesium levels. 

So, when we go to the aquarium store, make sure not to be a miser. We should get the best one in store despite the price.

These things are bought once or twice in a lifetime. So you better buy the best one. 

Water Change

We could do a water change with a lower magnesium salt mix if the level goes beyond 2000. The water change could be 60% to 90%. 

If there is any confusion about which salt mix to use, go for one with a low magnesium level. 

We should not get too impatient once we’ve done the water change. 

The next thing we want to do is have some patience and give it some time. The magnesium level will soon get down on its own. 

Use Chemical Methods To Lower The Magnesium 

We could use this method if we don’t prefer the natural method through water change. It’s advised if we look forward to lowering the levels quickly.

This is particularly preferred by those who have many types of corals in their reef tank. Corals are a bit too sensitive. 

Other than that, this method is not always recommended. This method causes the calcium level to rise. It also causes an increase in alkalinity in some cases. 

So, while doing this process, we need to use some good calcium test kits and alkalinity testers. This would help us to micro-monitor the calcium and alkalinity levels.

It is advised to eliminate any use of calcium supplements while we vacuum the aquarium off of magnesium. 

Now, let us get down to the main procedure. What we have to do is run the water in the aquarium through a reactor in calcium hydroxide.

The insoluble magnesium hydroxide in the water would precipitate this way. Hence, the water would be depleted of the extra magnesium. 

Then we need to filter the water through a filter sock with fine pores to collect the precipitate. 

Pro tip: If we have corals in our aquarium, use the best coral dips. Some are high in magnesium, thus increasing the level in the tank. 

Things To Avoid Getting Drastically High Magnesium Levels In Reef Tanks

Some fish foods or algae wafers that remain uneaten by fish dissolve in the water. These then, in turn, raise the magnesium level over time. 

So, use food sources that do not have an extremely high magnesium level.

Also, regularly doing water changes help to avoid the accumulation of magnesium from food sources. 

For your convenience, you could use some good algae wafers. Choose the one that suits your requirements most. A good algae wafer becomes a savior.

Finally, you’ll be able to maintain a low magnesium level in your tank.


Question: How does magnesium affect marine life?

Answer: Magnesium has many biological contributions to corals and invertebrates. It enhances their growth and development.

It does so by helping in bone and skeletal development. Besides that, it also helps to prevent excess precipitation of bicarbonate and calcium.

Hence, it also plays a significant role in keeping calcium levels in check. 

Question: What should magnesium levels be in a reef tank?

Answer: The ideal magnesium level in reef tanks should be between 1250-1400ppm. A magnesium level below the 1200 mark is bad.

If it exceeds the 1400 mark, it will get down naturally within a day or two.

So, don’t worry about that. We need to start worrying if the magnesium level goes beyond 2000ppm.

Question: Is magnesium bad for fish? 

Answer: Not. Magnesium plays two important roles in marine life. One is they help in their bone and skeletal development.

Another one is that they help to keep the calcium level in water within the recommended range.

Too high or too low calcium is also not good. The ideal range within which calcium should be kept is 395-425. 


So, are you enlightened about how to lower magnesium in a reef tank? We hope you are now ready to make your aquarium healthy again!

The process is not too complicated, so we hope you’ll be able to pull it off well.

Once you do, make sure that it does not happen again soon by using the correct food sources.