How To Soften Aquarium Water: Easy-To-Do Fool-Proof Methods

how to soften aquarium water
Japanese Fighting Fish is reader-supported. When you purchase through one of our links we may earn an affiliate commission (at no extra cost to you).

If your fish tank is in operation for more than a few months, you will likely need to know how to soften aquarium water.

Contrary to popular belief, routine water changes won’t always work because the water itself may not have an ideal balance of dissolved minerals.

As a result, maintaining soft fish tank water may require adding certain minerals as opposed to simply removing all of them through water changes.


dead fish in aquarium sand

Many people wind up with a tank of dead fish because they think using distilled water ensures they never have to worry about checking for soft water.

To prevent this from happening, you are much better off learning useful answers to the question “How to reduce water hardness in aquarium?”.

Fish, plants, and other aquatic creatures evolve in waters that have different kinds and amounts of dissolved minerals. As a result, they cannot live without the proper amount of minerals, or water hardness.

In a natural environment, rain, ground leaching, decay of shelled organisms, plants, and micro-organisms all help to keep water hardness fairly stable.

Some of these factors may be out of balance within the amount of water and room available in a home fish tank.

This is why the trend will always be towards an increase in general aquarium water hardness unless you take steps to reduce it. You will also need to use chemical additives to keep the water at a stable hardness so that drastic fluctuations don’t affect the well-being of the fish and other creatures in the tank.

Fun Fact: Read our article How Do Betta Fish Breathe? Fun Facts About Its Breathing to understand how important it is to have stable, clean, and well-filtered aquarium water.

Basics of Aquarium Water Hardness

aquarium with fish and decor

Pure water or “H2O” is composed of one atom of oxygen to two atoms of hydrogen per molecule. You only get pure water when it evaporates, leaving behind heavier molecules such as minerals.

When the steam cools and returns to the liquid form, it does not have minerals in it so long as the container is free of other molecules that will dissolve into the water.

In nature, water falling to the Earth in the form of rain already has some molecules in it.

Next, the water is absorbed by the soil and begins to shift around underground. Along the way to rivers, streams, and ponds, the water picks up certain essential minerals including calcium ions, magnesium ions, and carbonate.

There are no two places on Earth that have the exact same minerals, let alone amounts within the water table area.

We can, however, say that in general there is soft water, moderate, and hard water in terms of mineral content.

Soft water has very little mineral content, while hard water has the most.

In addition, the actual type and ratio of minerals dissolved have an impact on aquarium water chemistry stability.

General Hardness or GH refers to calcium and magnesium content in the tank. This will always go up over time unless you take steps to remove it.

Since tropical fish and others regulate their swim bladders and other biological processes using these minerals, it is very important to match what the fish is accustomed to.

Carbonate Hardness or KH refers to carbonate and bicarbonate dissolved in aquarium water. As aquariums age, pH also tends to go up. It is the KH that helps to keep the pH stable, especially as day to night and other natural rhythms cause pH fluctuations in the tank water.

When you take steps to reduce general hardness in the aquarium water, you may need to add other chemicals to increase KH so that you don’t cause excess disruptions in the pH levels.

Fun Fact: You could use the tips and hints presented in this article if you are planning on using saltwater. Read our post on Saltwater Tank Setup: How To Build One Based On 5 Factors to learn more.

Partial Aquarium Water Changes

changing water in aquarium

For many reasons, this is absolutely the worst way to soften aquarium water. Each time you do more than top off the tank to replace water lost from evaporation, you are asking for multiple kinds of water chemistry imbalance and other disruptions. These include:

  • Loss of KH, which can lead to massive pH swings. During the day, plants contribute differently to the pH in the tank than they do at night. When you don’t have the buffering of a robust KH level in the tank, it can lead to pH swings that kill the fish.
  • Loss of fish slime. As fish swim around in the aquarium water, they secrete oils much as your own skin does. This slime helps kill off harmful micro-organisms in the water, and also helps the fish feel safe.
  • Loss of beneficial bacteria free-floating in the aquarium water. Even though good bacteria tend to colonize the filter and substrate, there may also be some in the water. If the tank is small, even a minimal loss of good bacteria can lead to deadly ammonia and nitrite surges.
  • The new tap water may contain heavy metals, pesticides, or other pollutants. If you add an aquarium water conditioner, it can’t filter out every single harmful contaminant. This can only happen over time as the water moves through the filter.
  • Tap water run through reverse osmosis can have unpredictable effects. As reverse osmosis filter media is used, the amount of minerals removed will change. You should always test the tap water before adding it to the tank so that you know if the reverse osmosis affected the carbonate hardness of the tap water as well as the general hardness.

You can counter this by filtering any new soft water added to the tank for at least 24 with activated carbon before adding it to the tank. Just be sure to test for ammonia in case something gets into the new water that leads to a spike.

If you are determined to use water changes to soften water, never do more than 10% at a time.

Aside from massive disruptions, this will also help reduce the chance of swim bladder biochemistry-related problems and other issues that can kill most fish species.

As you learn more about natural answers to the question “How to reduce water hardness in aquarium?”, you will soon see that fewer water changes make for less work and healthier fish.

Growing Aquarium Plants

growing aquarium plants picture

When I first got started with aquarium keeping, the very idea of keeping plants underwater seemed strange and unappealing.

Back in those days, I liked the brightly colored plastic plants, and though they were safer and better for all fish species.

Very quickly, I learned these plastic plants led to all kinds of injuries and did absolutely nothing for the water quality.

It wasn’t long before I found myself solving just about every problem in my tanks by adding more live plants.

Softening aquarium water is no different. If you ask me “How to soften water in an aquarium?”.

One of the first things I will say is “add a few more plants”.

Aquatic plants are just like soil-based plants in the sense that they take up all kinds of molecules and store them in their leaves, stems, and other structures. This includes calcium, magnesium, and other minerals that they need to live.

Once the plant stores minerals in its leaves, stems, and roots; these molecules no longer circulate in the water.

All you have to do to keep the minerals from getting back in the water is trim out the leaves and stems before they die and decay.

How to Soften Fish Tank Water with Peat Moss

canadian sphagnum peat moss to lower ph of fish tank

Peat moss is a safe additive you can use to soften aquarium water. It will also lower the pH of the tank and give the water a light brown color.

Before you add it to the tank, you will need to boil it, and then soak it in water for a few days. This will help reduce the amount of staining from the tannic acids.

I tend to use peat moss in blackwater aquariums where I want both a lower pH and softer water to accommodate fish species that need both a low pH and soft water.

While I have also used it in a pinch for neutral to slightly alkaline tanks, it tends to be more trouble than it’s worth. In those cases, an ion exchange softening pillow is more effective and causes fewer problems.

Aquarium Water Softening Pillows

Most aquarium water softeners contain an ion exchanger that absorbs calcium and magnesium from the water.

As you can see in this video, the process is ongoing and constant, thus creating gradual change.

You can reuse them several times by soaking them in salt water, rinsing, and returning them to the tank.

For these pillows to generate soft water, they must pass through some kind of filter. If you are using a HOB filter, then you may need to install a bubble-up filter or some other additional filter for the aquarium water softening pillow.

Over the years, I have gotten these to work with HOB filters where I used DIY media for other components. For example, if the filter uses only cartridges, then I usually open them up and exchange the activated carbon for the ion exchanger within the water softener pillows.

Since I don’t use activated carbon past the first year of a tank’s life, it tends to be a simple matter of exchanging one media for another.

Fun Fact: Another issue that many aquarium owners face is aquarium water maintenance. To know more about this, read our article How Often To Change Aquarium Water: Best Methods And Reasons.

Frequently Asked Questions

How to soften water in aquarium?

There are several ways to reduce water hardness in a home aquarium. For the first year of operation, you will need to use water softening pillows or peat moss.

Later on, as plants take up more minerals in the tank, you may not need to use these additives as much, if at all for softening aquarium water.

A great deal depends on how many creatures are in the tank as well as how much mineral is being added from their waste and fish food.

You may also need to do partial water changes from time to time if the tank is heavily overstocked with fish or other animal-based creatures.

These water changes should always be accommodated by the addition of a KH buffer solution so that the pH doesn’t dip and surge unpredictably.

Even with these kinds of chemical filtration media, water changes tend to cause more problems than they solve because of decreased fish slime concentrations, and other disruptions.

It is better to manage increases in water hardness with routine testing and usage of water softening pillows or peat moss that have a slower, narrower, steadier impact on water chemistry.

Will fish species adapt to harder water over time?

If the change happens slowly enough, paradise fish and some other species of fish will adapt over time. This doesn’t mean that you can take a fish that is accustomed to soft water and let the water reach extreme levels.

What it does mean is that if the fish is in the tank for months to years and is doing fine, taking drastic steps can do more harm than good.

On the other hand, if you bring new fish home, you may need to keep the new fish in quarantine much longer than expected.

If the fish is used to very soft water in the store tank, then you will have to start there and let the general hardness reach the same level as in your main tank.

Failure to allow this adaption to happen naturally and slowly will most likely lead to illness or death for the new fish. To add insult to injury, if the new fish breaks out with some kind of infection, it will also spread to the other fish and may kill them.

Should I reduce water hardness rapidly?

Unless you are cycling a brand new tank with no fish in it, you should never attempt to rapidly adjust water hardness.

If the tank has been in operation for months to years, you can gradually shift the GH back to optimal parameters for the fish.

This is the single most overlooked factor when people move fish from one tank to another or decide to do a 100% water change. If you do not match the water hardness the fish are used to, it will kill them.


Unlike many other aspects of aquarium keeping, it can be challenging to use only natural methods to soften aquarium water.

Sadly, water that falls outside the proper hardness for the creatures in your tank can be a silent killer that you won’t recognize without doing routine water tests.

While some people will always recommend using water changes, I recommend using a water softening pillow or peat moss depending on the aquarium type and inhabitants.

Last Updated: July 12, 2022

Leave a Comment

Your email address will not be published. Required fields are marked *