Understanding water hardness in your aquarium

What is water hardness in aquariums?

Water hardness is a measure of the total dissolved salt content in water or the quantity of certain metallic ions. e.g. calcium and magnesium.

The general hardness (GH) refers to the total content of these combinations of salts and can be divided into carbonate hardness (KH) and permanent hardness.

KH can be removed through boiling the water, whereas permanent hardness remains after boiling. They’re often unavoidably introduced in new plants and blend into the aquarium environment.

How to test your water

You can test your waters’ hardness/softness by using a test kit – like our complete aquarium dip test kit. This all-in-one test kit offers a quick and simple way to test your tank’s water conditions. Simply enter one of the test strips into your water for a result.

Hard water will show a GH of 14 °dH and a KH of 10-15 °dH. This is generally acceptable for most community aquarium fish. Perfect for alkaline loving fish like African lake cichlids.
Very hard water will show a GH of 28+ °dH and a KH of 20+ °dH. This is too hard for most fish and may need to be treated or diluted with RO (reverse osmosis). RO is a filtration method used to remove impurities from the water.
Soft water will show a GH of 3-7 °dH and a KH of 3-6 °dH. This is acceptable for most fish, perfect for acid loving fish like neon tetra.
Very soft water will show a GH of 0 °dH and a KH of 0 °dH. If the water is too soft, it may lack essential minerals needed for fish.

There is no universal, ideal water hardness for an aquarium or fish tank. The perfect conditions are those, closest to your fish’s natural environment.

What happens if your aquarium water is too hard?

A high KH plays a significant role in the aquarium by keeping the pH stable. When fish breathe and your filter converts toxic waste, which releases hydrogen ions which can drop the pH. A high KH will soak these ions up like a sponge and stop the pH from changing too much, causing stress to your fish.

If your KH is very low, it’s likely that your water pH is acidic and could see fluctuations in your water parameters.

If you are keeping fish from the Amazon then you will want to have a lower water hardness. However, if you are keeping African cichlids or pond fish then a higher hardness would be better as this is what they are naturally used to.

What causes water hardness/softness?

Increased water hardness is caused by:

Where your tap water is sourced from – test the water coming out of your tap to see if it is hard or soft.
Calcium substrates like coral sand.
Some ornaments or pond walls which leach chemicals into the water.

Decreased water hardness is caused by:

Where your tap water is sourced from
Fish respiration and biological filtration over an extended period of time – Old tank syndrome
Plant growth

Why would you need to adjust water hardness?

Adjusting water hardness is essential for maintaining a healthy and thriving aquarium. Both fish and plants have specific requirements when it comes to water hardness, as it directly impacts their overall well-being.

If the water in your aquarium is too hard or too soft, it could lead to issues such as stress, illness, or even death for your fish.

Water hardness can be raised by adding a calcium based substrate or by adding a water buffering product. To lower water hardness levels perform small water changes with reverse osmosis or rain water.

How to soften your aquarium water

If you find that your aquarium water is too hard, there’s no need to panic. There are a couple of easy steps you can take to help soften your water.

1. Treat your aquarium

Once you’ve tested your water, you’ll need to perform several small water changes with reverse osmosis to bring the hardness down to a more suitable level for your fish. This will gradually help to reduce the hardness of the water over time.

You can use boiled tap water if RO water is not available.

2. Support your fish

Your aquarium water has a huge impact on the health of your fish, so they will need some support during the transition. You can help your fish by:

Using Aqualibrium First Aid Salt to help boost your fish’s ability to cope in stressful conditions.
Adding an air pump to your tank to increase oxygen levels – especially If you notice your fish gasping at the surface.

3. Add aquarium soil to your tank (optional)

Adding aquarium soil to your aquarium substrate can help buffer the pH levels in your tank, making it more stable for your fish. It could also help with plant growth. This method is purely optional and may not be necessary for all aquariums.

4. Add driftwood (optional)

Not only is driftwood good for naturally softening your water, it also adds a natural and aesthetic element to your aquarium. Driftwood releases tannins into the water which can help soften it over time as well as lowering pH. Just be sure to soak and rinse the driftwood thoroughly before adding it to your tank.

How to raise water hardness

Just as hard water can cause issues for your fish, soft water can also be a problem.

As mentioned, you can raise your tank’s water hardness by adding a calcium based substrate or water buffering product. You can also do the following:

To increase the hardness of your water, you can add commercial KH salts or do water changes using tap water.
Consider adding crushed coral or limestone to your aquarium, as it can help increase the mineral content of your water, making it harder.
Use the same treatment for your fish as you would for softening your water – Aqualibrium First Aid Salt or adding an air pump to your tank.

If you’re setting up your very first aquarium then you want to make sure you get everything right. Check out our starter guides for all the information you need to create the perfect aquarium for your fish.