How to Soften Aquarium Water? – 6 Fish-friendly Ways

how to soften aquarium water

If you are an aquarist or an aquascaping hobbyist, then you would have encountered the terms “hard water” and “soft water” at some point.

For both saltwater and freshwater aquariums, hard water and soft water are water qualities that play an essential role that directly impacts the health of the fish and aquatic plants and allows them to survive and thrive in their ecosystem.

Keeping in mind that the fish kept in aquariums may differ in species and origin, learning how to soften aquarium water and finding the ideal habitat for the fish is a little tricky, but not impossible.

What is Water Hardness in a Fish Tank?


In simplest terms, water hardness in a fish tank is a water parameter that is often described as the number of dissolved mineral contents, such as calcium and magnesium ions, that are found in the water.

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  • Hard water has high levels of mineral content, while soft water, on the other hand, has low mineral levels.
  • The pH level of a water is a quantifiable measure of how acidic or alkaline a solution is, and maintaining this in a constant level is vital to ensure the growth and health of the fish and plants.


Fish species must be considered when choosing the hardness or softness of the water in aquariums. Species like guppies, cichlids, and mollies are naturally accustomed to hard water, while gouramis, characters, and rasboras prefer soft water.

Therefore, you should check to either maintain or get rid of hard water in the fish tank to accommodate different fishes.

Ways to Measure Water Hardness


The most important parameters when measuring water hardness are Ammonia, Nitrite, Nitrate, pH, General Hardness, and Carbonate Hardness.

Among these, the two main parameters that should be checked to test are the GH and KH, which are often measured in degrees (dH) or as parts per million (ppm).

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  • General Hardness (GH) quantifies the amount of calcium and magnesium ions. Its measurement unit is dGH (Degrees of general hardness).
  • Carbonate Hardness represents the amount of carbonates and bicarbonate ions. It refers to the level of alkalinity of the water, with dKH ( Degrees of carbonate hardnes) as a measurement unit.


The presence of these minerals is what makes the aquarium water hard, less prone to pH fluctuations, and a suitable and stable environment for the fish and aquatic plants.

The table below shows the values for aquarium water hardness scale:

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General Hardness (dGH)Water hardness level Alkalinity level (dKH)
0 – 4Very soft water0 – 4
4 – 8Soft water5 – 7
8 – 12A little hard water7 – 8
12 – 18Moderately hard water9 – 12
18 – 30Hard water13 – 20
>30Very hard water> 20


Generally speaking, measuring the hardness of water is done through the total dissolved solids (TDS) which include minerals, salts, or metals.

The TDS shows the quantity of the total amount of organic and inorganic matters that are present, and will determine whether the aquarium water is of suitable quality. This can either be done using liquid tests or test strips.

  • Liquid tests can only measure one parameter at a time and can be done by combining aquarium water and testing solutions in a vial and then shaking it.
  • Unlike liquid tests, test strips can measure multiple parameters at once by dipping a piece of paper with multiple test pads in the aquarium water.

Why Do You Need to Soften Aquarium Water?


  • Meet the fish species’ ideal paramaters

When you are taking care of certain species like discus, gouramis, characins, and rasboras, it is critical to soften aquarium water to ensure their optimal growth.

While some of these fish may be able to adapt and manage to live in hard water, it still poses a threat to their overall health and well-being and will shorten their lifespan.

It is important for the fish to not only survive but to thrive in their fish tanks. Soft water species thrive in warm and acidic waters, and studies show that fish with no stress factors have intensified coloration and a boosted immune system.

  • Create ideal conditions for breeding

Activities like breeding will become problematic as well as the eggs cannot develop in an unsuitable water environment since thick egg shells would not be able to adapt. In the worst case scenario, the fish may not exhibit any kind of breeding behavior at all.

  • Limit algae growth

Another reason to soften the water is because it allows aquatic plants to grow stress-free which in turn minimizes algae growth, and helps aid the growth of the fish in the aquarium.

Ways to Soften Aquarium Water

There are a variety of options to choose from when reducing water hardness in aquarium tanks.

Before choosing which option you’ll take, it would be good to have a monitoring system of the pH, GH, and KH readings and see if there are notable fluctuations in these parameters. From there, you can decide if you will lower the hardness in fish tank water.

Below are 6 effective and fish-friendly ways for an aquarium hard water treatment:

1. Rainwater


If you’re looking for a cost-efficient and effortless way to fix hard water in a fish tank, then rainwater is your best bet.

Rainwater is the softest water available as it doesn’t contain the minerals found in hard water. If the rainwater is too soft, you can adjust this by mixing it with tap water.

The only challenge that comes with harvesting rainwater is, you have to take safety measures such as using sterile and food-grade containers to ensure that no chemicals will contaminate the water.

2. Distilled/Demineralized water


Another effortless way to soften water in a fish tank is by using distilled or demineralized water.

These have been purified to be neutral leaving no mineral content and a pH level of 7.0 making it an ideal method to reduce general hardness in an aquarium.

The only difference it has with rainwater is that using distilled/demineralized water is not cost-efficient, especially if you plan to use it in freshwater aquarium.

3. Water softening pillows


Water softening pillows are considered one of the best methods for softening water in small tanks.

They are sponge-like chemical filters that act as a magnet and attract potentially harmful minerals in the water which makes aquarium water softer.

These pillows work by forcing minerals like calcium and magnesium to pass through resin thereby reducing GH levels in the tank.

Much like using distilled/demineralized water, using water softening pillows are also costly as they need to be replaced every few weeks.

Though they can be recharged with the help of aquarium salt, it is only limited to up to 4 times.

4. Peat moss filters


A Peat Moss Filter is a soil-like substance and is a great alternative to water softening pillows.

It works by undergoing a process called chelation where it binds calcium and magnesium ions.

Simultaneously, it releases tannins and gallic acids which neutralizes carbonate and bicarbonate levels and help lower pH levels and gets rid of hard water in the fish tank.

The downside to using peat moss filters is that the tannins will stain the aquarium water brown. Soaking the filter beforehand can minimize this.

5. Driftwood


Driftwood naturally works if you want to fix water hardness in a fish tank. Much like peat moss filters, driftwood releases tannins.

Tannins are harmless to fish and can even boost their immune system to protect them from fungal infections. Driftwood like Mopani wood and Malaysian driftwood are good options if you want to decrease general hardness in an aquarium.

However, if driftwood isn’t decontaminated well and is not monitored for fungal growth, it can cause problems for the aquatic environment.

To avoid this, driftwood can be boiled to ensure that it is sterilized and to lessen the discoloration effect that it may have on the water.

6. Reverse osmosis deionization (RO/DI)


Reverse Osmosis Deionization (RO/DI) is a scientific way of lowering the hardness of aquarium water.

RO/DI does a good job if you want to treat hard water in a fish tank. It works by filtering the particles that go through it and trapping minerals and impurities, leaving only pure soft water.

However, this method is too potent, to the point that the water is too soft, making it unideal for the fish since they still need small traces of minerals to survive.

Like rainwater, water that underwent RO/DI can be mixed with tap water to make it suitable and balanced for the fish.


Water hardness is an important factor that should not be disregarded by hobbyists, and sufficient knowledge on how to soften aquarium water is vital.

Deciding whether the water of the fish tank will be hard or soft will depend on the fish species and aquatic plants that are being taken care of.

Water parameters should not only be ideal, but stable as well, as these are vital for the overall well-being and health of the fish that are being kept.

Extreme caution must always be exercised when altering water parameters to avoid stressing the fish and to gradually allow them to adapt to the environment.