How To Test Water Hardness In Aquarium Water

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It is essential to test the water hardness in your aquarium regularly, as this can have a significant impact on the health of your fish.

Without knowing the water hardness levels in your aquarium, it can be challenging to ensure that you are providing the best possible environment for your fish.

By using a simple at-home water hardness test kit, you can easily and quickly determine the water hardness levels in your aquarium.

This will allow you to make any necessary adjustments to ensure that your fish live in optimal conditions.

Read on to learn more about how to test the water hardness in your aquarium.

Why Is It Important To Test Water Hardness In An Aquarium

The most critical test for aquarium owners to perform is to test water hardness as often as possible.

When water hardness is not tested and tracked over time, it is virtually impossible to correct any problems in an aquarium system.

This test is so important because if there are any types of salts or minerals present in the water that is not being controlled, it can lead to a buildup of these minerals in a tank.

A buildup of salts and minerals will cause many stress-related issues for fish, corals, and invertebrates alike.

Testing Methods

There are several methods available to perform tests to test the levels of salts and minerals in an aquarium system.

Liquid Test Kits

One of the most common methods is using liquid test kits; liquid test kits are straightforward to use and give accurate results in under 5 minutes.

Digital Testers

Another method available to aquarists for testing water hardness is by using electronic or digital testers.

Digital testers can be somewhat expensive, but they allow users to perform many chemical tests with only a couple of drops of water.

How Easy Is It To Test Water Hardness?

Testing for water hardness is relatively easy.

If using a liquid test, all that has to be done is add a few drops, place drops of the test solution to some tank water, time how long it takes, and watch the color change occur.

Using a digital tester is as easy as sticking the probe into the aquarium and waiting for a reading.

Keep in mind that water hardness can affect the pH levels, so it is essential to test for both at the same time.

How Often Should Aquarium Water Be Tested For Hardness?

Testing saltwater and freshwater tanks can occur as often as once a day or once every few months, depending on the system being maintained.

Some reef aquarists have to test water hardness as often as possible because it can fluctuate daily.

General Hardness (GH vs. Carbonate Hardness (KH)

Two types of hardness can be tested in aquarium water- general hardness (GH) and carbonate hardness (KH).

  1. General hardness measures the total amount of dissolved minerals in the water.
  2. Carbonate hardness measures the number of bicarbonate ions (HCO3-) present in the water.

Carbonate hardness is critical to test because it helps control the pH levels in an aquarium.

What Is Total Alkalinity (TA) vs. Carbonate Hardness (KH)

The difference between TA and KH is that Total alkalinity measures all the buffers in aquarium water, so it is also essential to test for. These are carbonate hardness, bicarbonate hardness, and borate hardness.

Although KH levels can be used as an indicator of pH, KH usually measures “carbonate hardness,” which is not always an accurate indicator of pH.

Both total alkalinity and carbonate hardness are essential to monitoring the successful maintenance of aquariums because they help control the pH levels in an aquarium system.

How To Test General Hardness (GH) In Aquarium Water

  1. Pour the test solution into a clear glass container.
  2. Fill the container ¾ of the way with aquarium water.
  3. Add drops of the test solution to the aquarium water.
  4. Wait for at least 6 minutes before reading the results.

How To Test Alkalinity Or Carbonate Hardness (KH)

  1. Get a test kit. You can buy aquarium water testing kits at most pet stores.
  2. Fill a clean container with water straight from your aquarium.
  3. Add drops of the aquarium water testing kit reagent until a distinct color change occurs.
  4. Compare this to the color chart that comes with your kit to determine the carbonate hardness level in your tank.

What Are The Recommended Levels For GH And KH For An Aquarium?

The minimum levels of GH and KH needed in an aquarium are not well established, but most experts agree that 5-12 degrees (Fahrenheit) for GH is sufficient.

  • The optimum level of GH in an aquarium depends on the type of fish you keep.
  • For soft water types like livebearers, the best range to aim for is 2-6 degrees (F) GH.
  • For slightly hard to medium water types like tetras, barbs, and small catfish species, aim for 4-12 degrees (F) GH.

For tough fish like cichlids or marine species, aim for 6-18 degrees (F) GH.

Can You Raise Or Lower The Hardness Of Your Aquarium’s Water With Additives?

Yes, you can. There are additives available at pet shops that will increase or decrease the hardness of your aquarium’s water.

To do so, simply follow the directions on the additive packaging.

Here are a few ways you can soften your aquarium water if needed.

Are there any risks associated with adjusting water hardness in an aquarium?

Yes. If you decrease GH and KH levels significantly, the fish in your tank may be stressed and more susceptible to disease and infection.

On the other hand, if you increase GH and KH levels excessively, they can cause pH crashes that exacerbate fish diseases and stress your fish.

Which Brands Produce Reliable Tests For Water Hardness?

We recommend three brands of GH and KH tests that are reliable.

  1. API
  2. Seachem
  3. Just Fitter


Testing water hardness is an essential step in maintaining a healthy aquarium.

In this article, we have outlined two methods for testing GH and KH levels in aquarium water.

We have also discussed the importance of both total alkalinity and carbonate hardness in helping to maintain pH levels in an aquarium system.

Finally, we provided some tips on testing, raising, or lowering water hardness using additives.

So, whether you are just starting with keeping an aquarium or a seasoned pro, it is always a good idea to regularly test your water’s hardness.

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