The Effect of Water Hardness on Aquariums & Fish

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Any fishkeeping enthusiast knows the importance of reducing nitrates and ammonia in a freshwater aquarium. But what about water hardness? How might your water’s dissolved minerals content affect the quality of your aquarium water and the health of your fish and plants?

We’ve shared everything you need to know about the effect of water hardness on aquariums in this guide.

πŸ“Œ Key Takeaways:

  • Water hardness is a measure of water’s pH and mineral content.
  • There are two water hardness measurements for fish tanks: general hardness (GH) and carbonate hardness (KH).
  • Water hardness has effects on your aquarium fish, plants, and tank.

πŸ€” What Is Water Hardness?

Let’s start with a brief overview of water hardness.

Water hardness is a measure of the amount of calcium and magnesium ions a water supply contains.

Soft water has low levels of hardness minerals, while hard water has high levels of hardness minerals.

The hardness of a water supply depends on a number of factors, including:

  • Geological composition
  • Seasonal weather patterns
  • Water source (groundwater vs surface water)
  • Runoff

Most normal tap water supplies are hard, meaning that they contain excess calcium and magnesium.

Hardness and pH are interconnected. If your water has a low pH, it likely also has a low hardness. So, you can boost the water’s pH by increasing hardness.

Don’t confuse general water hardness with carbonate hardness.

In aquariums, general hardness (GH) is a measure of minerals in the water. Most fish need these minerals for metabolism regulation, bone and exoskeleton growth, and more.

Carbonate hardness (KH), or alkalinity, is a measure of carbonate and bicarbonate compounds in the water. These compounds prevent pH fluctuations by acting as buffers.

Hard water vs soft water for aquarium and fish

πŸ“ How Is Water Hardness Measured?

General hardness is measured in degrees, either dGH (degrees of general hardness) or Β°GH. You might also see general hardness measured in parts per million (PPM). 1 dGH is equal to 17.848 PPM.

Carbonate hardness is referred to as KH or alkalinity, and is measured in dKH (degrees of carbonate hardness) or Β°KH.

πŸ§ͺ How To Test For Water Hardness In An Aquarium

You can test for water hardness in an aquarium with a specialized aquarium water hardness test kit.

Most commercial test kits will give you two readings: a general hardness reading and a calcium hardness reading.

We recommend a liquid test kit as the most accurate testing method.

To use a liquid test, fill a glass test tube with water from your aquarium and add a couple of drops of the test solution to the tube. Put the cap on the test tube and shake. The liquid will change color, and you can compare the color with the included color chart to get a water hardness reading.

Alternatively, you can use test strips to take a reading of your water hardness. Just dip a strip in a sample of water taken from your aquarium and wait for it to change color. Compare this color to the color chart to get a hardness reading.

Using the titration method to determine water hardness

🚱 How Does Hard Water Affect Aquariums And Fish?

There are a few ways that hard water might affect aquariums.

Hard water minerals form limescale, which could build up on your aquarium components, like your filters and pumps, reducing their efficiency.

Hard water could also affect your fish, if your aquarium houses fish that aren’t tolerant of water with a high mineral content.

As with any other water parameter, if the total hardness is too high for your fish, then the fish may be unable to thrive in their water conditions.

πŸ”Ž Does Water Hardness Affect Aquarium Plants?

So, water hardness might affect fish – but what about plants?

Most aquatic plants are adaptable to water hardness and thrive in both soft and hard water. Magnesium, calcium, and other hardness minerals don’t have an adverse effect on plant growth. However, hardness does affect pH, and maintaining optimal pH levels is essential for plant growth.

High pH may prevent nutrients from dissolving into the water column, which affects plant growth, since most aquarium plants rely on these minerals to stay healthy.

If your tank water hardness is high, we recommend water wisteria, Amazon swords, and cryptocoryne, since these plants are more tolerant to higher pH and require fewer insoluble nutrients.

⚠️ Is Hard Water Dangerous To Aquarium Fish?

Hard water is only dangerous to fish if the fish doesn’t tolerate that level of hardness.

Like all water parameters, water hardness isn’t found at similar concentrations in all the natural water supplies across the world.

That means certain fish are used to certain water hardness levels, just as different fish have different pH tolerances.

So, a fish that’s used to and adapted to hard water in the wild won’t be harmed by hard water in the aquarium.

But, fish that are used to softer water in their natural environment may experience growth problems as a result of being housed in a hard water tank, and probably won’t live as long as they should.

That’s why it’s essential to do your research before you buy any kind of freshwater fish for your tank. Check the fish’s tolerable range of general hardness and carbonate hardness before adding it to your tank.

hard water in fish tank

🐠 Fish Species That Tolerate Hard Water

Looking for hard water fish species for your tank?

Here are a few fish to consider:

  • African cichlids, congo tetras, flagfish, Odessa barbs, some rainbowfish – Tolerate hard water (12-18 Β°GH/200-300 PPM)
  • Most African cichlids and some Central American cichlids – Tolerate very hard water with a high mineral content (18+ Β°GH/300+ PPM)

You can also consider mollies and platies if your water isn’t too hard, as long you maintain stable conditions.

If your water is very hard and you don’t want to put your fish at risk, we recommend primarily considering African cichlids, which are native to Lake Victoria and Like Malawi – both known for their rocky habitats and high water hardness.

🐟 Which Fish Species Prefer Soft Water?

On the other end, if you plan to buy the following aquarium fish species, keep in mind that these fish prefer softer water or water that’s only moderately hard:

  • Banded panchax – Requires very soft water (0-3 Β°GH/0-50 PPM)
  • Angelfish, tetras, South American cichlids, corydoras – Require soft water (3-6 Β°GH/50-100 PPM)
  • Betta fish, mollies, platies, and other livebearers – Require moderately hard water (6-12 Β°GH/100-200 PPM)

If you’re uncertain whether a specific fish species will tolerate your current aquarium conditions, speak to your local breeder and ask for their expert advice.

Betta fish in soft water fish tank

πŸ“ˆ How To Increase Aquarium Water Hardness

Let’s start by saying that maintaining stable water parameters is more important than constantly trying to achieve optimal conditions, which could upset your water balance and stress your fish.

Keep in mind that adjusting your water hardness will cause your water’s pH level, carbonate hardness, and calcium and magnesium levels to be altered.

But if you have softer tap water and you’re keen on keeping fish that thrive in hard water, here are a few ways you can boost hardness in your aquarium.

Water Additives

There are several commercial additives that you can buy specifically for increasing water hardness in aquariums.

Just search “aquarium water conditioner” on Google to see which products are available. Follow the instructions on the bottle carefully to add the right amount of product for your intended results.

Crushed Coral

Adding crushed coral to your tank is an easy way to boost both general hardness and carbonate hardness.

You can encourage the crushed coral to circulate in your tank by adding it to an area of fast-flowing water, like your aquarium filter or pump.

The biggest issue with this method is that it’s difficult to know exactly how much coral you need to raise your water hardness by an adequate amount.

Crushed coral and limestone increases  hard water in aquarium


Limestone is another material that can be used to boost water hardness.

Just as limestone rock in a natural environment causes water hardness to increase, so does limestone in a home aquarium.

You can place limestone features in your fish tank to help raise general hardness and KH. Limestone will gradually release minerals into the water, gradually raising the hardness.

How quickly will your hardness rise? That depends on your other water parameters, including the water’s acidity.

πŸ“‰ How To Soften Aquarium Water

What if you need to soften the water in your aquarium? What methods can you use for this purpose?

Here are the best ways to soften aquarium water.

RO Water Treatment

If you need to achieve very soft water in your fish tank, you can use a reverse osmosis or distilled water system to remove all the minerals from water, therefore reducing its hardness.

Reverse osmosis/distillation produces virtually pure water, but most aquarium fish – even those that prefer very soft water – still need minerals to survive. So, you may need to remineralize your water before using it in your aquarium.

how to lower water hardness in aquarium with an RO system

Aquarium Soil

Some aquarium soil substrates act as a buffer, keeping water’s pH below 7 and reducing total hardness. As an added bonus, the soil will provide the nutrients that your live plants need for healthy growth.


Driftwood gradually releases tannins into the water, so it’s another good item to add to your tank if you want to lower water hardness and pH.

We recommend soaking the driftwood in boiling water before adding it to the tank, which will sterilize it and prevent it from tinting the water brown.

πŸ“‘ Final Word

Many freshwater fish thrive in hard water conditions. However, if you want to raise healthy fish that live for years in your home aquarium, you need to check their recommended water parameters before you spend your money.

As with other elements of water chemistry, your water’s hardness and pH should remain stable and consistent to prevent stress to your fish. Maintaining general hardness will ensure that your fish can thrive in a safe and stable environment.

Don’t forget about other important determiners of aquarium water quality, like ammonia, nitrate, nitrite, and pH.


Is hard or soft water better for fish?

Not all tropical fish require the same hardness levels in aquarium water, so it depends on the species. Fish that are used to naturally soft water, like banded panchax, angelfish, and tetras, will do better in softer water in your tank (such as reverse osmosis water). Fish that are used to slightly hard to very hard water, like African cichlids, congo tetras, Central American cichlids, and rainbowfish, will thrive in harder water in your tank.

What causes hard water in a fish tank?

The water in your aquarium might be naturally hard if you’ve used a tap water supply, since tap water typically has a moderately high to very high hardness. Or, certain items or decorations in your tank, such as crushed coral and limestone, might be gradually increasing hardness.

How do I get rid of hard water in my fish tank?

You can get rid of hard water in your aquarium by treating the water with an RO water filter, or adding driftwood or aquarium soil to the tank.

  • Laura Shallcross
    Senior Editor

    Laura is a passionate residential water treatment journalist who holds an undergraduate degree in Print Journalism and a master’s degree in Creative Writing. Over a span of 5 years she's written on a range of topics including water softening, well water treatment, and purification processes.

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