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How to Adjust Water Hardness in the Aquarium

How to Adjust Water Hardness in the Aquarium

Written by: Tammy (@aquarist_tland Team Buce Plant


Most aquascaping hobbyists have heard the importance of testing specific water parameters like Ammonia, Nitrate, and Nitrites. But when it comes to water hardness, many new aquarists don't test for it, or even know it exists!

Water hardness is a parameter that is commonly forgotten about, even though it can play an essential role in the health of the aquatic specimens being kept.

In this article, we’ll be answering the following questions:

1. What is water hardness?
2. How do you measure water hardness?
3. Why should I adjust my aquarium water hardness?
4. How do I raise my aquarium water hardness?
5. How do I lower my aquarium water hardness?


What is Water Hardness in the Aquarium?

Water hardness refers to the number of dissolved minerals found in water. In nature, water gets its hardness from limestone or dolomite sources. As the water runs over these types of rocks, they accumulate minerals from them. Tap water is typically hard and may need to be softened for certain freshwater aquariums. Some aquarists will use aquarium driftwood, Catappa leaves, or other natural botanicals that release tannins, creating a Blackwater Biotope.

When it comes to your aquarium, general hardness refers to the number of minerals, like calcium and magnesium ions, in the water. These minerals are beneficial for many fish and invertebrates because they help with their metabolism and help strengthen their bones and exoskeletons.

Your aquarium's water hardness, or general hardness, is also important because it has a direct relationship to water pH and can act as a buffer. For example, if you would like to either raise or lower your pH level, you will first need to adjust the hardness.

Most freshwater fauna can easily adapt to hard water. Aquarium plants can also thrive in a range of general hardness. However, if your water hardness goes to extreme ranges, algae is more likely to thrive and outcompete your aquatic plants.

 aquarium water hardness scale

How Do You Measure Water Hardness in the Aquarium?

A very general way to measure the hardness of water is through its TDS (total dissolved solids). TDS is a term used to define the measurement of the total amount of organic and inorganic matter present in a particular volume of water. The higher the TDS is, the harder the water.

For aquariums, the main parameters we want to measure are the GH and KH:

1. GH (General Hardness)

The GH level of water refers to the number of dissolved minerals, mainly calcium and magnesium, that are present in the water.

2. KH (Carbonate Hardness)

The KH represents the alkalinity of the water sample. It is a measurement of the buffering capacity (i.e. the water’s ability to prevent swings in pH). Carbonates and bicarbonates are able to neutralize acids that are in the water with them, so the more carbonates found in a sample of water, the less likely the pH will shift. If the KH is very high, the water’s pH is likely to be very basic. If the KH is very low, the pH of the water is probably acidic and is also more susceptible to dramatic swings.

  • Note: Carbonate hardness can be measured in two ways- dKH (degrees of KH) or ppm (parts per million.)


Reasons to Adjust Water Hardness

1. Match Ideal Parameters for Specific Species

The fish that we keep in our aquariums or that we see at our local fish stores come from all around the world. Each fish’s original location has its own set of water parameters. For example, tropical fish from the Amazon River prefer soft and acidic water, because the Amazon River is full of tannins that help it maintain such low levels of pH and hardness.

In most cases, it's unnecessary to match your tank's parameters to the exact parameters of a fish species' ideal environment because the majority of the fish available in stores have been bred in captivity. Basically, these fish have most likely already adapted to being able to live in waters that have different parameters than their ideal.

altum angel fish

If the fish is wild-caught (i.e. came straight from its original location in the world), then you should consider trying to adjust the hardness if it’s drastically different from its origin. For instance, if you tried keeping a wild-caught Altum Angelfish or a wild Discus in hard water, you may notice that they might not do well because they’re used to living in very soft water. The fish may get stressed and be more prone to catching diseases.

However, most freshwater fish can easily adapt to hard water. Snails and shrimp can also benefit from hard water because the calcium helps strengthen their shells and exoskeletons. Make sure to research the best range of parameters for that specific shrimp, because too much hardness can cause problems with their molts.

Although, keep in mind that if your water is considered "very hard," your choice of aquarium stocking is limited to African cichlids. This species originates from lakes with high hardness levels, and will actually thrive better in harder water.

 freshwater guppies

2. For Breeding Purposes

Livebearers like guppies, platies, and mollies are prime examples of fish that do much better in hard water versus soft water. This is because the females use minerals like calcium to give birth to large batches of fry at a time, and they need to be able to replenish it. Otherwise, they may develop bent spines from the lack of available calcium in the water.

On the other hand, certain fish species need to be bred in soft water. It’s been said that rams (Mikrogeophagus ramirezi) are unable to breed in very hard water because the calcium causes the eggs to harden to the point that sperm is unable to penetrate the membrane. Hard water may also cause the eggs to have issues with developing.

 black ram breeding

3. To Grow Specific Plants

Luckily, most aquatic plants are fine in either soft or hard water conditions. There are just a few exceptions where some prosper better under certain parameters. For example, several species of Vallisneria are unable to do well in soft water. They may even just melt after a while if the water is too soft.


4. For the Safety of Your Tank's Inhabitants

Remember, your aquarium's water hardness has a direct relationship to water pH. Hard water usually has a high pH, and soft water usually has a low pH. The high mineral levels in hard water will act as a buffer to reduce the acidity of water, meaning raise the pH.

Many areas in the US have soft tap water (resulting in a low pH) with a very low KH measurement. This results in a very low buffering capacity and an aquarium that is susceptible to large swings in pH, which can be dangerous for the aquarium's overall health. These large swings in water parameters can also potentially crash the cycle, which could end up being fatal to the tank’s inhabitants. So, aquarists using very soft tap water should add some carbonates into their aquariums as a safety net to prevent any pH crashes.

 freshwater shrimp

Methods of Adjusting Water Hardness

Now that we know what water hardness is and the reasons why we might want to adjust it, here are ways to increase or decrease the water hardness in your aquarium.


How to Increase Water Hardness in the Aquarium

1. Use Water Additives and Products

There are popular products available to help boost the GH and KH for aquariums. SL-Aqua Black MORE GH Conditioner and SL-Aqua Black MORE KH Conditioner are examples you can use to increase the hardness of your water. Just make sure to follow the directions and base the dosages on accurate readings for the water you’ll be using it on. These products also work well for remineralizing RO/DI water.

Some aquarists use Epsom salt, which contains magnesium, in their aquarium. However, it is difficult to measure the appropriate dosage, and Epsom salt lacks calcium so calcium levels will need to be adjusted in other ways. Regardless, this method is NOT recommended because it can have negative lasting effects on your fauna.


2. Add Limestone-Based Rocks

Limestone is a source of calcium carbonate and having it in the aquarium will allow it to release minerals into the water. The more acidic the water is, the quicker the limestone will mineralize the water. This process should gradually slow down and eventually stop as the water gets more basic. Fortunately, there are limestone-based aquascaping stones that are available so you are ensured that they’re safe for aquariums. Two rocks that are known to raise water hardness are Seiryu Stone and Pagoda Stone.

 seiryu stone

3. Add Other Sources of Calcium Carbonate

Adding crushed coral or crushed oyster shells can be used to increase an aquarium’s water general hardness and carbonate hardness. Placing some into your filter where water can constantly flow through them will give you the quickest results. They can also be used as part of the substrate of the tank. Another option that is available in the aquarium market is aragonite, a substrate that is known for helping to keep African cichlids.

However, keep in mind that it can be difficult to add the exact amount needed to raise the water hardness to the desired level.


How to Decrease Water Hardness in the Aquarium

1. Use a RO/DI System

A reverse osmosis deionization system filters out all the particles in the water that goes through it, even stripping away the minerals. By the end of the process, it should produce nearly pure H2O, leaving you with the softest water possible. Pure water like this would actually be bad for any fish though because they need minerals to survive, so aquarists will remineralize RO water before adding it into the aquarium. Some may even mix it with regular tap water to reach their targeted water hardness level.

Check out SL-Aqua's GH conditioner and KH conditioner to help replenish necessary minerals removed by RO/DI systems.


2. Add Aquarium Soil

Planted tank substrates like Controsoil will buffer the water and keep the pH of water below 7, lowering the total water hardness. It will also provide all the nutrients necessary to grow plants!


3. Add Driftwood

Driftwood contains tannins that will release over time while it’s submerged underwater. These tannins will soften the water and lower the pH. When tannins are released into the water, they may tint it brown to resemble tea. To avoid this, you can boil the driftwood before adding it into the aquarium. Boiling it also ensures that it is sterilized and safe to place into the tank.


4. Add Other Sources of Tannins

Other sources of tannins include: Catappa/Indian almond leaves, alder cones, and peat moss. Each of these should release tannins to help lower pH and hardness levels. Learn more about the use of tannins in Blackwater Biotope - Tips & Tricks.

blackwater tannins


Water hardness is often disregarded in the aquarium hobby, but it is arguably an essential parameter for maintaining a stable and safe environment for your fish. Always keep in mind that while there are ideal water parameters for your fish, stability is most important. Quick swings in pH can be detrimental for your fauna, so express caution when working to change parameters.

While chasing ideal water parameters is usually not recommended, it may be necessary under specific circumstances. I hope you found this article post helpful in achieving your aquarium goals! 


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Christopher L Barkley - June 12, 2024

Very interesting and informative article, found out the exactly the information I needed! Thanks

Beth - June 12, 2024

Very Helpful. One of the few thorough articles I found concerning water hardness. My poor beta is in need of some calmer water (he’s becoming a little less playful/active). This will help me adjust the pH slower to find his optimal levels.

Belinda - August 31, 2023

Great information thanks!

Alison - March 3, 2023

This site is very helpful! I have a 27 year old goldfish who has been lethargic lately, the only condition that has changed is we got a new household water softener, which I suspect is set to regenerate too frequently. I brought Goldie a few buckets of artesian water which we are fortunate to have in town, and he responded well. I will check his ph, add a few oyster shells to the filter, and dial back the softener. Thanks for the info!

Alice - October 12, 2022

The pH of my water is my last parameter I’m working on stabilizing. Love the tips!

Tia - October 12, 2022

This article was very helpful! Our city water is really soft. I didn’t know of any way to change it. I’ll start testing it out to see if I can achieve the hardiness level I want.

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