How To Reduce Water Hardness In A Fish Tank? The Best Guide!

Last Updated on April 29, 2022 by Marco C.

Have you ever learnt how to reduce water hardness in a fish tank?  This skill is very useful for any aquaponics or fish tank owner.  Read our article to learn the best information on this topic.

Learning even the smallest amount of water chemistry will make your life a lot easier.  Do not be overwhelmed by the task.  There are some basic concepts you should grasp that will help you a lot in the long run. 

One of the aspects that is important to learn about is water hardness.  Simply put, water hardness is related to the number of minerals present in your water.

 If there are too many minerals, it can be dangerous for living things.  This means your fish and plants’ life may be threatened.

According to McGill University in America, “ In general, water with less than 60 ppm can be considered soft, water with 60-120 ppm moderately hard, and water with greater than 120 ppm hard.” There are special tools to measure the hardness of your water.

It is really important to consider the source of the water you use to fill up your tank.  Oftentimes, city tap water is enhanced with more minerals.  Using water like this will affect the health of your fish tank.  

One of the main minerals found in water is calcium.  The calcium comes from limestone that is often present in people’s wells. If you have a very hard water source, you will need to know how to reduce water hardness in a fish tank.  

You came to the perfect place! Keep reading to learn the key things you need to know!

How Does Hard Water Affect My Fish?

The presence of minerals in water is completely natural.  However, when this gets out of control, it can be dangerous.  There are a few reasons for this.

First off, when the water hardness is high, the PH of the water is affected.  PH is one of the most important things to consider when dealing with a PH fish tank.  This is because the PH affects many other aspects of your tank’s water chemistry.

As minerals increase in your water, the PH level will increase.  This means the water becomes more alkaline.  Although there are certain fish that may prefer a slightly alkaline environment, most of them prefer a neutral PH of around 7.  

The PH of your water also affects your plants.  In aquaponics, water from the tank is used to water and fertilize your plants.  If you have alkaline water, it will then alter the PH levels of the soil.  

Just like fish, plants most often prefer a neutral PH.  So basically, if your water is too hard, it will affect the health of both your fish and plants.  When your water is hard, you cannot use other methods to lower the PH.  

The high mineral content prevents the PH from being altered.  It acts as a buffer that blocks efforts to reduce PH levels.  When the PH levels are off, the whole fish’s ability to function is compromised.  

PH levels will affect the fish’s ability to metabolize food, develop healthy growth, and in some cases will cause death.

Read more about: How To Fix Hard Water In Fish Tank

How Can I Lower The Water Hardness In My Tank?

The easiest and fastest way to reduce water hardness in a fish tank is to do a water exchange.  This means that you need to empty some of the water in your tank and replace it with new water.  However, you need to understand the hardness of the new water.

In order for this method to work, the new water must be “softer” than the current water.  If you have a severe water hardness issue, you will need to replace all the water in your tank.  When the problem is not out of control, you may be able to solve it by doing a 30-40% water exchange.

A potential good source of water to do this is rain that you collect from nature.  It is always best to test the water before putting it into your tank.  Keep in mind that rainwater can also contain a lot of pollutants from the air.  

Be sure that your new water is clean and balanced.  

Another way to reduce water hardness in a fish tank is to create a peat moss filter.  Peat moss is decomposed moss that comes from swampy areas.  Lucky for us, Peat moss is able to capture minerals like calcium and magnesium.  

You can try to pass your tank water through some peat moss to see if it reduces the water hardness enough.  Peat moss is available for purchase at many pet stores.

Peat moss is available for purchase at many pet stores

Read more about: How To Start Seeds For Aquaponics?

As you can see there are many different methods to reduce water hardness in a fish tank.  Choose your solution based on what resources you have available and the severity of your problem. 

Feel free to comment and ask questions below!


Will hard water kill fish?

If your water becomes too hard it can threaten the life of a fish. In severe situations, your fish will die. This is because hard water affects many aspects of your tank’s water, such as the PH. When PH levels get too high, your fish is unable to perform vital functions crucial to their survival.

How can water hardness be reduced?

Water hardness can be reduced by adding “soft” water into your tank. You can purchase special de-ionized water that does not contain any minerals or use rainwater. In some serious condition, you may need to empty all the contents of your tank and do a 100% water exchange. If your problem is less severe try to start with only exchanging part of your water contents. Continue testing the hardness levels until you reach a healthy level.

Is hard water good for aquariums?

Hard water is not good for aquariums and should be avoided at all costs. Always inspect your water source to make sure it does not contain an abundance of minerals such as calcium and magnesium.

What causes hardness in fish tank water?

The presence of minerals causes water hardness in a fish tank. Most often these minerals are calcium and magnesium. Most tap water sources have some traces of minerals in it.