Everything You Need to Know About Water Hardness in Aquarium

Everything You Need to Know About Water Hardness in Aquarium

Water hardness plays an important role in aquarium keeping. Besides parameters like temperature, pH, and ammonia, the amount of total dissolved minerals is equally crucial in determining how your plants and fish will respond. Any mineral above the normal value may disrupt the ideal range of water hardness.

In this regard, how to soften aquarium water becomes one of the concerns of aquarists. Luckily, there are many ways to bring water hardness to the optimum range, such as chemical filtration, deionization, driftwood water softening, etc.

In this article, we will learn some of the safe and effective ways to soften aquarium water and learn all the if’s and but’s that revolve around water hardness.

What is Water Hardness?

Water hardness indicates the concentration of dissolved minerals in aquarium water. While there could be plenty of minerals in excess and traces, water hardness is majorly determined by the level of calcium, magnesium, and carbonate.

The higher the concentration of calcium, magnesium, and carbonate, the harder the water is.

Water hardness is categorized into general hardness and carbonate hardness based on the following-


  1. General hardness (GH)- The concentration of dissolved calcium and magnesium in the aquarium water.
  2. Carbonate Hardness (KH)- The concentration of carbonate and bicarbonate ions in aquarium water. Top of Form


Both these parameters are measured in parts per million (ppm).

Water in natural habitats has varying levels of water hardness. It could be as low as 10 mg/L (Amazon River) or as high as 500 mg/L (African rift lakes). Therefore, fish that thrive in natural habitats require the same level of hardness in their aquarium, too. It is because their body is physiologically adapted in the same manner.

Fish that thrive in hard water find it difficult to cope up in soft water and vice versa. Therefore, it becomes essential to replicate the same water parameters in the aquarium for fish’s health and survival.

Even if the fish survive in varying water hardness, it is impossible to breed and spawn them in contrasting conditions.

How does water become hard?

As already mentioned, the more minerals are present in water, the greater its hardness. As water moves through streams, certain rocks, such as dolomite and limestone, keep increasing their hardness and alkalinity. Likewise, soil alters hardness by increasing the dissolved minerals.

As the water follows its natural path, the minerals keep adding. By the time it reaches the tap at home, the hardness could have increased multiple times. Even though increased hardness may not cause problem for human consumption, it can deteriorate fish health as they are highly susceptible to water paraments. Therefore, using tap water directly into the aquarium may cause problem.

Below is the guideline that will help determine the GH and KH of water

Water Hardness General Hardness (GH) Alkalinity (KH)
Very hard >30 dGH >20 dKH
Hard 18-30 dGH 13-20 dKH
Moderately Hard 12-18 dGH 9-12 dKH
Slightly hard 8-12 dGH 7-8 dKH
Soft 4-8 dGH 5-7 dKH
Very soft 0-4 dGH 0-4 dKH


Determining water hardness 

The first thing you need to determine water hardness is a water test kit. These kits contain test strips that have chemicals in them. As you dip the strip in the water, the chemical present in it will react with the mineral present in water, leading to a change in color.

Once the color changes, you can compare it with the chart on the packaging that will tell you the range of water hardness.

General Hardness (GH)– This will give an idea about the dissolved calcium and magnesium in the water. The water is considered soft if the value is less than 8 dGH and hard when the value is 9 dGH or more.

 Carbonate Hardness (KH)– It indicates alkalinity of water that also gives an idea about water’s pH. If the value of KH comes higher, it means the water has high resistance to changes in pH value; thus, will stay stable. On the contrary, low KH value will indicate that water is susceptible to rapid changes in pH value.

Once you know the GH and KH of your tap water, you can look for fish that have almost similar ranges and can easily withstand such values. However, if you have other fish in mind that cannot survive in hard water conditions, you can make efforts to soften it.

Ways to soften aquarium water

There are different ways to reduce water hardness in an aquarium. Here, it is important to know that maintaining a stable water parameter is more important than running behind in getting the ideal value. Also, if you are softening the water, ensure the process is slow and gradual, as sudden fluctuation could do more harm than help.

Below we have listed some safe and effective ways that will soften the water and make it suitable for fish that require so.

  1. Using Rainwater 

Rainwater is naturally soft; therefore, it can be used while performing water changes in the aquarium to soften it. In this method, you need to collect rainwater and check its GH and KH levels. If it’s too soft, mix some tap water to bring it between the range.

While collecting rainwater, ensure you do it in a sterile container that doesn’t leach any chemicals.

  1. Using Water Softening Pillows

Water-softening pillows consist of resins that can absorb minerals like calcium and magnesium present in the water. Additionally, they can also absorb soluble heavy metal ions.

The best thing about these chemical filtration media is they can be reused to bring down the general hardness of aquarium water. Once you have used the pillows, soak them in brine water solution for 3 hours and they’ll become ready to reuse.  

  1. Using Peat Moss Filters

Peat moss has the ability to bind ions such as calcium and magnesium. While the ions are absorbed, peat moss releases tannins and gallic acid. This exchange of minerals contributes to softening the aquarium water.

One downside of using a peat moss filter is it can make the water slightly yellow. To avoid it, boil peat moss for 2-3 minutes in water and then soak it in clean water for a few minutes. While boiling will ensure that peat moss is devoid of all parasites, soaking will not turn the water yellow or brown.

Peat moss filter can be used to soften aquarium water in 3 ways-

  1. Soaking peat moss in a clean container to soften water.
  2. Use peat moss in your filter as filter media.
  3. Adding peat moss into your tank as it is, but only after boiling.


  1. Driftwood 

Driftwood also softens the water the way peat moss does by introducing acids that neutralize the carbonates. Additionally, driftwood also adds brown tinge to the water, but luckily only for a short span.

Driftwood releases tannins in the water that aid in reducing water hardness. Tannis do not pose any harm to the fish, in fact they can boost their immune system and protect them from fungal infections.

One thing you need to keep in mind while adding driftwood is it should be boiled and decontaminated beforehand. Escaping this step allows parasites and other microbes to enter the aquarium and cause a mess. At the same time, if the aquarium is not properly aerated, driftwood can develop fungus. So, proper monitoring becomes essential.

  1. Reverse Osmosis 

Reverse osmosis is a process of water purification in which water is purified through demineralization or deionization to keep it safe for fish. A semipermeable membrane is used in the process that captures 99% of impurities. Besides this, the membrane also blocks minerals like calcium and magnesium, making it soft. Installing an RO setup becomes essential if you have a large aquarium.

  1. Indian Almond Leaves

When excess minerals are stressing fish, using a natural product, Indian almond leaves come to the rescue. The presence of tannins in the leaves binds to the minerals and reduces their hardness. The three benefits of using Indian almond leaves to soften water are

  • They reduce water hardness naturally.
  • The antibacterial properties of Indian almond leaves make the water free of bacteria and thus provide a healthy ambiance for fish.
  • Addition of leaves bring more natural look to the tank and mimics the atmosphere of streams and lakes for fish.

Using almond leaves is very simple. All you need is to collect some fresh leaves and wash them properly to prevent infestation of any parasite in the water. Now dry the leaves under the sun. Once they turn brown, introduce them inside the tank and let them settle at the bottom.

You can check the water hardness after a week, which would have dropped considerably.

  1. Distilled Water

If nothing from the above seems handy, just replace some of the tank’s water with distilled water. Distilled water is readily available in most of the stores and you can get it easily. Since there are no minerals in distilled water, there is zero hardness. Replacing some of the tank’s water with distilled water will drop the hardness and bring it to the desired level.


Aquarium water hardness is essential not only for fish but aquarium plants too. Fish and plants that are kept outside their normal water hardness range show adverse effects. Those who survive only in hard water will not survive in soft water conditions and vice versa.

Since every aquarium fish is not too flexible when it comes to water parameters, especially nitrite, nitrate and carbonate content, keeping everything within limit is essential. Levels above or beyond the range will put the tank’s habitants in stress that may lead to their death.

So, either you have to make efforts to bring water hardness within the range of tank’s habitants or stock species that are suitable for your tap water’s hardness. Remember that you need to keep an eye on GH and KH of water and if anything seems disoriented, aim for gradual changes.

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