How to Soften Aquarium Water?

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Soft and hard water refer to various minerals that are present in the water. Hard water contains more minerals, while soft water contains fewer minerals. Different fish species prefer a certain type of water. So, you may have to soften the aquarium water at some point. There are several ways to do that!

I’ll talk about it in this article and tell you more about how you can soften aquarium water efficiently. I’ll also tell how softer water benefits the fish and offer a few examples of fish species that prefer hard and soft water.

Keep reading to find out more!

What is Water Hardness?

Water hardness is defined as the total amount of dissolved minerals contained in the water. In other words, hard water is rich in minerals, especially magnesium and calcium sulfates, carbonates, and bicarbonates. In contrast, soft water contains very little calcium or magnesium.

Water hardness can be measured using different metrics. The total molar concentrations of Ca2+ and Mg2+ are expressed with various units, the most common being parts per million (ppm), millimole per liter (mmol/L), degrees of general hardness (dGH), or grains per gallon (gpg).

The most common metrics used are dGH and ppm, so these are the ones you’re most likely to see when reading about water parameters. Now, as I’ve already mentioned, soft water contains just a small quantity of minerals, but that doesn’t mean 0 minerals. In fact, both soft water and hard water contain calcium and magnesium, just in different quantities.

There are some general thresholds we use when classifying water as hard or soft. And there are also different degrees of water hardness. Moderately hard water has a dGH value somewhere around 3.38–6.74. Regular hard water sits at roughly 6.75–10.11 dGH, while very hard water measures more or is equal to 10.12 dGH. Soft water has the lowest dGH at around 0–3.37.

5 Ways to Soften Fish Tank Water

Not all fish species require the same water parameters. Some fish thrive in hard or very hard water, while others need soft water to be healthy. But what if you have a soft water fish species and you only have access to hard water? Luckily, there are multiple ways to soften hard water and make it more hospitable for your fish.

There are plenty of water softening products on the market. These products result in chemical reactions that precipitate the calcium and magnesium in the water, getting rid of them and thus reducing the concentration of minerals. However, there are also some lesser-known ways of softening hard water which I’m going to discuss below.

– Rainwater

Rainwater is practically free and makes an eco-friendly option for softening hard water. All you have to do is increase the ratio of rainwater to hard water in the aquarium. Rainwater is naturally soft, so you won’t have to worry about testing its GH levels.

However, you might want to test its pH. Regular rainwater has a slightly acidic pH of 5.0-5.5. If you live in a polluted area, rainwater might even drop to a pH of 4.0. These values might not be appropriate for your fish species.

When performing water changes using rainwater, the water in the aquarium might become too soft for certain fish species. In that case, you could try using a water hardener or adding some tap water after performing water changes.

As a side note, you need to be cautious when collecting rainwater. You’ll have to ensure that the water is clean and free from contaminants. The container you use to collect the raindrops should be sterile and food-safe so that you won’t get any microplastics, dyes, or other chemicals leaching into the water.

All this being said, rainwater is probably not going to be your first option, because you need a large volume to perform water changes. You can’t always rely on rainwater, because precipitation levels vary wildly throughout the year.

– RO Water

Short for “Reverse Osmosis”, RO is a quick and easy process of water purification. While you might need to make an upfront investment, this will save you a lot of money and time in the future. You’ll always have soft water on hand, whenever you need it for replacements!

But how do RO filters work? These reliable devices use a high-pressure pump to force water through a semi-permeable RO membrane. The membrane filters all the large molecules in the water such as mineral salts, leaving up to 99% of all the impurities behind.

As a result, you get rid of 95-99% of all the calcium and magnesium, so you get soft water every time you use this filter. This is an excellent product to have on hand if you don’t want to do a lot of prepping and planning or if you have a large capacity tank and need a high volume of soft water.

– Driftwood

This method is both affordable and fun. By using driftwood, you can soften the aquarium water with almost no effort, but you also decorate your fish tank at the same time. You can choose from a variety of shapes and sizes of driftwood for all aquarium types.

This method softens the water through a chemical reaction. When soaking in the water, driftwood leaches tannic acid. Tannic acid then interacts with carbonate compounds and neutralizes them, contributing to a lower concentration of calcium and magnesium salts. However, there’s one downside to this method.

The tannic acid will stain the water a reddish or brownish color. This compound is the same thing that gives the tea its specific dark color. Your aquarium might look a little weird. On the plus side, tannins aren’t only harmless to fish, but they can even have health-promoting effects. These compounds strengthen your fish’s immune systems and help them fight against fungal infections.

Driftwood can also be prone to fungal growth, though. So, it’s a double-edged sword. You have to pay attention to it and clean it regularly. When you first introduce driftwood into the aquarium, you should make sure it’s clean and sterile. Otherwise, you might be introducing harmful bacteria or toxins into the tank.

– Peat Moss

Peat moss will directly reduce the water hardness by binding magnesium and calcium ions. This process is called chelation, and it’s an easy way to soften hard water through demineralization. Peat moss also releases gallic acid and tannins, which lowers pH and KH levels.

However, before you use peat moss as filter media in your aquarium, you should boil it for a couple of minutes. This is to prevent any parasites or bacteria from hitchhiking the peat moss. You can also soak the peat moss into clean water after boiling it.

Now, you can use peat moss to soften the hard water using three methods. The first one implies the soaking of peat moss in a container filled with clean water. First, you need to decontaminate the peat moss. After you’ve boiled it, put the moss in a clean container and fill it with tap water.

Let it sit there for 2-3 days and use that water to soften your aquarium water when you do water changes. You can also test the pH, KH, and GH levels in the tank to keep all the factors constant.

The second method is the usage of peat moss as filter media. You can put the peat moss into the tank’s filtering system. The water flow will do the rest, but you should follow a few steps before that.

Make sure you wrap the peat moss in a mesh bag and boil it to remove contaminants. Let the peat cool down in a container with cool water. Then, take the peat moss bag out and put it inside the filter behind one of the filtering pads. You can test the pH levels to manage the softness of the water closely.

Finally, you can add the peat moss to your tank as is. You’ll need to boil it, soak it, and pack it in a filter bag, though. If the bag receives enough water circulation, that’s all it needs to soften the hardwater.

– Distilled Water

This one is a simple method that only requires mixing the distilled water with tap water and then pouring it into the aquarium water. Distilled water has had its minerals removed, so it’s very similar to soft water.

But never use only distilled water when changing the water in your aquarium because distilled water has no minerals. Your fish will die in distilled water because they can’t get the necessary nutrients and minerals.

However, with good remineralization, distilled water becomes one of the best types of water for your aquarium. So, if you’re having problems with hard water in your aquarium because of high levels of minerals, using distilled water can balance things out. But you should regularly check the pH, KH, and GH levels when using this method.

Why Should You Soften Aquarium Water?

Every species of fish has certain requirements when it comes to aquarium water. Some will only thrive in soft water while others need the mineral concentration of hardwater. If you keep a fish in a substandard type of water, it may become sick or even die. That’s why, when you buy fish from a shop, they’ll give you aquarium requirements.

Even though most fish species will eventually adapt to the hardness of the water, it doesn’t mean some won’t become sick and die. Why force your fish to go through a traumatic experience when you can soften or harden the water however you see fit? The methods to do it are simple enough.

To assess whether water is hard or not, use the dH (General Hardness) indicator. If it’s between 0 and 6 and the ppm (parts per million) is between 0 and 100, the water is soft. If the dH is between 6 and 25 and the ppm is between 101 and 449, the water is slightly hard.

Lastly, if the dH is above 30 and the ppm is above 450, the water is hard. Wild-caught Discus, for instance, need soft water, so water conditions with a dH of 23 and ppm of 250 aren’t suited for them.

Why Does Aquarium Water Become Hard?

Aquarium water will become soft over time because the GH (general hardness) and KH (carbonate hardness) in the aquarium will drop naturally. The minerals are naturally depleted because the fish will consume them. Then, organic waste and plant respiration will also deplete the KH levels by adding acids into the mix.

That’s why you perform water changes to an aquarium, aside from cleaning the waste. You need to replace the lost minerals and keep the same water softness or hardness level. Tap water isn’t the best type of water you can use because it’s softness or hardness may be inadequate.

There’s another perspective, as well. Water will evaporate over time, and if there aren’t enough fish to consume the minerals, they’ll keep piling up in a decreasing body of water. This means that the remaining water becomes heavy due to having more minerals lying around.

How to Test Water Hardness?

While water hardness might seem difficult to manage at first sight, the process is simple enough. Testing water hardness is quick and easy, and you can do it in less than 15 minutes. All you’ll need is a water sample and a water hardness testing kit.

Most kits are in the range of $4-10, and they’ll last you for multiple uses. These kits come with a color chart and multiple testing strips. After selecting the water sample, just insert one testing strip and wait for the strip to change color. It should take a few seconds.

Then, you consult the color chart in the kit to see the hardness level. Each color included in the chart corresponds to a certain range of values. These values are typically measured in gpg (grains per gallon), and these values correspond to different cutoff points of water hardness levels.

The cutoff points for the water hardness in gpg are as follows: between 0-3.50 gpg for soft water, 3.56-7.01 gpg for moderately hard water, 7.06-10.51 gpg for hard water, and more or equal to 10.57 gpg for very hard water.

If you find it easier to work with dGH or ppm values, you can also use the following conversion chart. Just multiply the gpg value you get from the test by the corresponding dGH or ppm value in the table. For example, if your testing kit shows you got a value of 8.5 gpg, the equivalent in ppm would be 146 (8.5 * 17.12 = 145.52).

  1 gpg 1 dGH 1 ppm
1 gpg 1 1.043 0.05842
1 dGH 0.9591 1 0.05603
1 ppm 17.12 17.12 1

 Hard Water Fish Species

As you may already know, each fish species has its specific water quality requirements. Some fish need hard water to be healthy, while others prefer soft water. You cannot mix hard water and soft water species in the same community aquarium. That’s because, inevitably, some of the fish will suffer due to suboptimal water parameters.

If you want a colorful aquarium with different fish species, you have to ensure that they’re compatible. Part of that is thriving in the same water conditions. To get a better idea about what fish species you can keep together, I’ll give you a rundown of hard water and soft water fish species.

Starting with the hard water species, the most popular ones include:

Keeping only hard water fish species like these is a great way to bypass the need to soften aquarium water.

Soft Water Fish Species

Most fish species prefer hard or slightly hard water. Soft water fish are rarer, and usually more difficult to keep. That’s because soft water is less stable. The low mineral concentration makes soft water prone to rapid pH changes. Without a lot of calcium ions in the water, the pH is less likely to remain alkaline.

Because soft water tends to become acidic, you’ll have to test its pH and perform water changes more often. On the other hand, soft water makes aquarium cleanup easier. Without so many minerals floating around, you’re less likely to get limescale buildup on corners, filters, or other aquarium components.

Although rarer, soft water species do indeed exist. Keep in mind that even soft water fish won’t do well in water that’s too soft, though. All fish still need a certain level of minerals in the water. As long as the concentration of minerals doesn’t drop too close to 0, you can safely keep a bunch of colorful species such as:

  • Gouramis
  • Angelfish
  • Most Tetras and Characins
  • Corydoras Catfish
  • Rasboras
  • Barbs
  • South American Cichlids
  • Certain oddball fish like the Red-stripe Halfbeak, the Dwarf Puffer, Malayan Freshwater Pipefish, or Armoured Stickleback.

You’ve probably noticed that I’ve also listed a few of these species as hard water fish. The truth is that these species are naturally suited for soft water, but they’re highly adaptable. They can live equally well in hard water.


Water hardness is an important parameter to watch when setting up and maintaining your fish tank. While most fish species thrive in hard water, some fish require soft water. If you intend to keep a soft water fish aquarium, there are multiple ways you can soften hard water to make it more inhabitable for your pets.

By far the best methods are RO filters, adding distilled water, or using natural alternatives like driftwood or peat moss. But remember, softer water isn’t always better. Most species can adapt to higher water hardness, but no species will be healthy in water that’s too soft and void of minerals.

It’s important to strike the right balance. At-home water hardness testing kits can help you monitor the mineral concentrations in the water and adjust accordingly.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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