All About Aquarium Water Hardness (And How to Lower — and Increase It)

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Do you know the difference between hard and soft water? Aquarium water chemistry can be pretty intimidating for beginner fish keepers, but it’s really important to know the basics.

It’s easy to feel overwhelmed by all the technical terms, but don’t worry, you don’t need a lab coat and goggles for this lesson – we’re going to break it down super simple in this beginner’s guide!

So let’s get started about aquarium water hardness!

Key Takeaways

  • Water hardness is a measure of the dissolved minerals in your water.
  • Tap water from different parts of the world has different total hardness/ general hardness (GH) and carbonate hardness (KH) levels.
  • GH is a measure of calcium and magnesium ions, while KH is a measure of calcium carbonate anions. pH is a measure of the acidity or basicity of the water.
  • Water hardness affects the kind of tropical fish, invertebrates, and plant species you can keep in your aquarium.
  • It is possible to adjust water hardness, but it is easier to choose fish species that are adapted to your local water.

What Is Water Hardness?

Water hardness is the concentration of minerals dissolved in the water. More specifically, it refers to the amount of calcium and magnesium in the water, as well as trace amounts of other minerals. You can check out our lastest video from our YouTube Channel. We go into more details in the blog post below. Be sure to subscribe if you enjoy our content!

The concentration of these minerals depends on the geology of the area where the water originates. That means water hardness varies from area to area, and it all depends on the kinds of rocks the water passes over before it gets to your home. Dolomite and limestone are common rock types that alter water chemistry.

You can get a general idea of the hardness of your tap water in the US by checking out the United States Geological Survey (USGS) website1. However, testing your aquarium water quality and parameters at home is the best way to get an accurate picture.

We can break it down one step further and measure general hardness (GH) and carbonate hardness/alkalinity (KH), and both can be very important.

Read on to learn everything you need to know about water mineral levels.

General Hardness (GH)

General hardness measures the concentrations of mineral salts in your aquarium water. These minerals are magnesium (Mg) and calcium (Ca) ions, and they can be measured in degrees (dGH) or parts per million (ppm).

A water GH level of 4-8dGH or 70-140 ppm is a suitable level for most freshwater fish tanks. However, this depends on the type of fish, invertebrates, and plants you keep.

Carbonate Hardness (KH)

Carbonate hardness refers to the measure of carbonate and bicarbonate levels in the water. This determines your water’s buffering capacity against pH crashes and swings. Unstable water quality can be very dangerous to many fish, so this is an important parameter to consider.

Water pH levels change slightly throughout the day but within a certain range. This is a natural process, but it is dangerous if it happens too fast.

The higher the KH, the higher your pH is likely to be, and also, the more stable your pH level will stay. On the other hand, a very low KH is typical in a tank with a low pH which is prone to fluctuations.

However, a higher KH is not always ideal. High carbonate hardness makes it more difficult to adjust your pH levels. This can be a problem if you want to change your water parameters to keep a certain species of fish.

When it comes to carbonate hardness, 3 dKH is a safe level, although many commercially bred aquarium fish are adaptable between 1 and 10 dKH.

Does Water Hardness Really Matter?

Different aquatic plants and fish species have evolved to live in the type of water found in their part of the world. The minerals that make water hard or soft are very important for healthy fish and plants, but the amount they need varies between species.

Many aquarium fish are very adaptable to water hardness, especially when captive bred. However, it’s still a good idea to match the preferred natural habitats and water chemistry of your fish. Some species have very specific requirements and just won’t do well in the wrong water.

African Cichlids in a Rock Aquarium

Most fish do well in slightly hard water, and many can adapt to even harder water parameters. However, few species will thrive in very hard water. African cichlids are a notable exception and a great example of fish that need very hard water.

As you will learn later in this guide, you can change the water hardness in your aquarium to suit the types of fish you want to keep. However, it might be easier to match your fish to the kind of water you already have.

How Does Water Hardness Affect Fish?

Water hardness affects osmoregulation in fish. This impacts their ability to regulate salt and water in their bodies. Outside of their tolerance range, fish can become very stressed or even die.

Carbonate Hardness (Kh)

Earlier in this post, we learned that a low carbonate hardness (KH) allows your pH value to swing drastically, which can be stressful for your fish.

Carbonate Hardness does not have the same physiological effects as general hardness, but the buffering capacity is key to maintaining a stable pH level and environment for your pets.

General Hardness (GH)

Fish need minerals, just like we do. Without them, normal bodily functions like growing and reproducing would be impossible.

So, we know that incorrect carbonate and general hardness can be dangerous for fish, but what about the other life in our aquariums? Does hard water affect invertebrates and plants too?

Low general hardness can be very dangerous for aquarium invertebrates. It can cause problems for molting shrimp and cause thin, weak shells on snails.

Very soft water can also be dangerous for live aquarium plants, although many plants are also sensitive to high carbonate hardness. Some plant species, like Vallisneria, show a definite preference for harder water.

How to Measure Your Water Chemistry

You probably know that you should measure your aquarium water parameters regularly to be sure you are performing enough regular aquarium maintenance. However, aquarium water testing also allows you to monitor other important parameters like hardness.

Unfortunately, simply testing your pH levels is not enough. pH, KH, and GH are often linked in nature, which means that water with a low pH tends to have a low GH and KH too.

However, tap water can be different. Therefore, it is important to test both GH and KH, since they aren’t necessarily linked to pH.

The good news is that the process is pretty much the same as measuring other water quality parameters like nitrates. Let’s take a look at the basic process of testing your hardness levels.

Strip Tests

Strip tests are super easy to use. Some all-in-one kits can be used to measure multiple parameters, including pH, GH, KH, nitrates, and nitrites.

API Test Strips

This is a basic test kit that are used in many pet stores. There are better options, but this is readily available at most stores

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Simply dip the strip into your aquarium water and compare the colors on the strip with the example card to read your water parameters. Some aquarists even cut the strips in half to save a few bucks, but be sure to follow the instructions on your product for the most accurate results.

Liquid Test Kits

Liquid test kits are also very easy to use and even more fun! These tests use droplets of colorful liquid to measure the GH, KH, and other parameters of your aquarium water.

Reading the Results

We can put water hardness into different categories from very soft water, all the way up to ‘liquid rock’! Let’s take a look at five basic categories and their general hardness levels:

  1. Very soft water: 0-4 dGH /0-70 ppm

Very soft water has low levels of dissolved calcium and magnesium. Most plants and animals will not do well in these conditions.

  1. Soft water: 4-8 dGH/70-140 ppm

Many South American and Southeast Asian fish thrive in softer water. Softer water is also ideal for many freshwater plant species.

  1. Moderately hard water: 8-12 dGH/ 140-210 ppm

Moderately hard water is a safe environment for most of the popular freshwater aquarium fish in the hobby.

  1. Hard water: 12-18 dGH/ 210-320 ppm

Livebearers and other fish that enjoy moderately hard water can often adjust to hard water conditions.

  1. Very hard water: 18-30 dGH/ 320-530 ppm

Very hard water is often known as ‘liquid rock’ because it contains high concentrations of minerals. Most freshwater fish will not thrive in very hard water, although cichlids from Central America and East Africa prefer these parameters.

What Fish Are Good For Hard (And Soft) Water

Let’s take a look at some popular examples of hard and soft water fish that you might want to keep in your home aquarium.

Hard Water Fish Species

Soft Water Fish Species

How To Change Your Water Hardness

It is possible to change your water hardness, but that is not always your best option. Stability is important, and when you start having to adjust levels, there is always the chance of human error.

Inconsistent water parameters can be stressful for your fish. However, if your water is naturally soft and you want to keep African Cichlids, you’re going to need to increase your water hardness.

Increasing Your Water Hardness

Increasing general hardness is a matter of adding calcium and magnesium salts to the water in a safe and manageable way. Before you adjust your water hardness, make sure you have tested your GH, KH, and pH level.

Let’s take a look at some simple methods of increasing water hardness.

  • Crushed Coral/Coral Sand

Adding crushed coral is a great natural way to increase your water hardness. However, it is not easy to reach an exact level using this method.

You can add crushed coral directly to your substrate or use it in your aquarium filter. It will act faster if you place it in a mesh bag in your filter where there is a constant flow of water.

  • Limestone Rock

Another way to gradually increase your tank water hardness is to use certain types of rocks as your hardscape. Rocks like Texas Holey Rock are beautiful limestone-based rocks that are very popular. This rock is high in calcium carbonate, so it will increase the pH, Gh, and KH of your water over time.

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Texas Holey Rock Natural Limestone

With its ability to raise pH and hardness, this rock is an excellent choice for African Cichlids

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  • Commercial Water Hardeners

Purpose-made products like Seachem Equilibrium are available for managing your water hardness levels.

Similarly, products like Seachem Alkaline Buffer are available for managing your carbonate hardness levels. These products are great because they allow more precise control.

Decreasing Your Water Hardness

Decreasing your water hardness can be achieved by reducing the concentrations of magnesium and calcium salts in the water. Read on to learn how to soften aquarium water.

  • RO/DI Water

It is more difficult to soften aquarium water if you have very hard water. One way to achieve this is to start over with pure water and add the minerals that you need. Reverse osmosis deionized (RO/DI) water is neutral and contains no minerals.

You can make your own RO/DI water at home using a reverse osmosis system. Buying one of these systems is a small investment, but they will pay for themselves in the long run.

This pure water is extremely soft and has a neutral pH. It is not safe for your fish or plants until you have remineralized it with something like Seachem Equilibrium or mixed it with tap water.

  • Distilled Water

Distilled water can be used in the same way as reverse osmosis water. However, you will still need to increase the total hardness of the water with magnesium and calcium carbonate compounds before it is suitable for keeping fish.

A simple way to remineralize distilled and RO/DI water is to cut it with tap water. Simply add small amounts of tap water to the pure water until the desired water hardness level is reached.

Your water will become harder over time if you are not performing regular water changes and simply topping up your tank with tap water. Test your water regularly and use the correct mixture of distilled and tap water each time you top up your tank to keep your levels stable.

  • Water Softening Pillows

Water-softening pillows are another great option, and they are very easy to use. These pouches use a resin that attracts magnesium and calcium to remove them from the water.

You will have less precise control when using this method, but it is very easy and works well for soft water fish like tetras and angelfish.

  • Peat Moss

Peat moss can be used to lower pH levels in your fish tank. This organic material will leach tannins into the water, staining it yellow or darker.

Peat moss can be effective for creating blackwater freshwater aquariums and softening your water to some degree, but it is not very effective as a water softener if you have very hard water.

  • Aquarium Soils and Driftwood

Some aquarium soils, like Controsoil, can also effectively soften water, making them a great choice for planted tanks. Driftwood, almond leaves, and other plant materials that leach tannins and stain the water will lower the pH and hardness somewhat.

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What Causes Water Hardness In Aquariums?

Water hardness is the amount of dissolved calcium carbonate anions, and magnesium and calcium ions in the water. The hardness of water varies depending on where you live in the world and the kinds of rocks your water flows over before reaching your home.

Does Aquarium Water Hardness Matter?

Aquarium water hardness can be very important. Most aquarium fish are quite adaptable, but some species have specific requirements for survival and breeding.

What Should Water Hardness Be For An Aquarium?

The ideal water hardness varies depending on which kind of fish you keep. Most aquarium fish and plants will thrive in water with a general hardness (GH) of 4-8 degrees or 70-140 parts per million.

What Should Water Hardness Be For An Aquarium?

The ideal water hardness varies depending on which kind of fish you keep. Most aquarium fish and plants will thrive in water with a general hardness (GH) of 4-8 degrees or 70-140 parts per million.

Does Water Hardness Affect Aquarium Fish?

Water hardness has some important effects on a fish’s ability to live and grow, so fish can become sick in water that is too soft or too hard. Some fish will survive in water with an incorrect hardness but will not spawn or breed.

Final Thoughts

Understanding water hardness is important, especially if you want to keep fish or plants with specialized requirements. I recommend testing your local water parameters before you set up an aquarium. That way, you can choose the perfect species to match your conditions.

However, it is possible to adjust your water hardness with the right materials, techniques, and a little practice. Just be sure to make your changes slowly and carefully to avoid stressing out your fishy friends!

Do you have a hard or soft water fish tank? Tell us about your experiences in the comments below!

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