Can Hard Water Kill Fish: Is It Harmful to Aquarium Life?

Hard water can have a dramatic and deadly effect on fish in aquariums. But what causes this effect and can it be avoided? In this article, we’ll explore the effects of hard water on fish and discuss how to prevent any lasting damage.

At its most basic level, hard water is just water with too many minerals dissolved into it. These minerals can react with other chemicals in the aquarium, compromising the delicate balance that all living things require to survive. When added to an aquarium, it can disrupt the pH levels or even act as a poison to some species of fish.

Hard water can cause significant health issues in fish if gone untreated for too long; nausea, lethargy, stress, and eventual death may occur. Thankfully, there are ways for you to get rid of these minerals without removing your entire tank’s worth of water so you can keep your aquatic friends healthy and happy!

Can Hard Water Kill Fish

What is Water Hardness in the Aquarium?

Water hardness is an important factor to consider in your aquarium as it can affect the health of your aquatic life and plants. The hardness of water is linked with the presence of specific minerals, such as calcium and magnesium ions.

It is typically created when water passes over limestone or dolomite sources, or through the use of aquarium driftwood, Catappa leaves, or other natural botanicals that release tannins. The water hardness also has a direct relationship with pH levels and can act as a buffer if you wish to adjust either up or down.

Most freshwater fauna can easily adapt to hard water but extreme ranges may lead to algae growth that outcompetes aquarium plants. Therefore, it is important to make sure water hardness levels are within suitable ranges for aquarists’ needs and preferences. Additionally, adjusting these levels can be beneficial for certain fish and invertebrates since minerals help support their metabolism, bones, and exoskeletons.

Will hard water kill your aquarium fish?

It is important to understand the ideal water parameters for your fish in order to ensure their health and well-being. Total hardness is an important factor, and it is important to know the ideal range for your particular species of fish. Very soft water may not be suitable for most aquarium fish, with the notable exception of the clown killifish (Epiplatys annulatus).

Tetras, angelfish, Corydoras, and South American cichlids prefer soft water. Meanwhile, most tropical fish such as mollies, platies, and bettas prefer moderately hard water. It’s important to monitor your tank’s water conditions closely and make any necessary changes to their environment or diet in order to keep them healthy and happy.

It is essential to remember that every aquarium is unique, with its own set of parameters based on lighting and temperature. Keeping your tank within the desired ranges for each water parameter will ensure a healthy environment for your aquatic friends.

When deciding what type of water to keep in an aquarium, it’s important to consider the needs of your fish. Generally speaking, most moderately hard water fish species such as platies and mollies can tolerate slightly harder water.

However, African cichlids, Odessa barbs, flagfish, and certain rainbowfish families prefer hard water ranging from 12-18 GH/200-300 ppm. For very hard water conditions of 18+ GH/300+ ppm, African cichlids are likely the best suited for this environment, with various Central American cichlid species also being able to tolerate this type of water.

These ranges are generally accepted guidelines within the hobby but may vary depending on individual preferences. Therefore, it is important that you research your fish’s specific needs before introducing them into their tank in order to ensure their health and safety.

What Fish Do Well In Hard Water?

Most freshwater livebearers are able to adapt to water with a higher hardness. However, once the hardness levels become too high, the stocking of your aquarium is limited to African cichlids endemic to certain lakes such as Lake Malawi and Lake Victoria, as well as some Central American cichlids.

It is also noteworthy that while most aquatic plants can tolerate a wide range of general hardness levels, some hobbyists have reported increased growth of certain types of algae in more extreme conditions, which can lead to an unhealthy aquarium environment for aquatic plants.

For these reasons, it is important to adequately research the different requirements for each type of fish or plant before introducing them into the tank.

How Do You Measure Water Hardness in the Aquarium?

The hardness of water can be measured using TDS (Total Dissolved Solids), GH (General Hardness), and KH (Carbonate Hardness). TDS is a 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. GH refers to the number of dissolved minerals, primarily calcium and magnesium, that are present in the water.

Finally, KH gives an indication of the alkalinity levels of a sample of water and how it will affect pH levels. (i.e., its buffering capacity.) KH can be measured in two ways: dKH (degrees of KH) or ppm (parts per million).

Reasons to Adjust Water Hardness

Adjusting water hardness is an important aspect of fishkeeping and can make a huge difference in the health and well-being of your fish. Hardness levels can be adjusted by using reverse osmosis (RO) membranes or adding minerals to the tank to achieve ideal water parameters for different species.

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Match Ideal Parameters for Specific Species

When choosing fish for your aquarium, it is important to consider the natural water parameters in their original environment. For most freshwater fish, hard water levels are suitable and they can easily adapt.

However, wild-caught fish that come from soft water environments may not do well if placed in hard water tanks. It is also important to research the specific needs of shrimp since too much hardness can lead to issues with molting.

African cichlids, on the other hand, will actually prefer harder waters as this is their natural environment. Ultimately, you want to do your research so that you can provide your fish with the most suitable environment possible for them.

For Breeding Purposes

When understanding the needs of fish, it’s important to research the type of water that is best for each species. Livebearers, such as guppies, platies, and mollies, must have hard water in order for them to be able to replenish essential minerals like calcium so they can give birth to their fry successfully.

In contrast, some fish like rams (Mikrogeophagus ramirezi) need soft water in order to breed since too much calcium can lead to fertilization issues with their eggs. Therefore, it’s important to make sure that you provide your fish with the correct type of water for their particular species in order for them to survive and thrive.

To Grow Specific Plants

When choosing aquatic plants for your aquarium, it is important to consider the hardness of the water in your tank. While many aquatic plants can do well in either soft or hard water conditions, there are some species that should only be kept in specific water parameters.

Vallisneria, for example, does not do well if the water is too soft and can even melt away over time. It is important to research the specific requirements of each plant before introducing them to your tank.

Additionally, make sure to check the pH and temperature of your aquarium regularly and adjust as necessary to maintain optimal conditions for your fish and plants.

For the Safety of Your Tank’s Inhabitants

It is important to consider water hardness and pH when setting up an aquarium. Hard water usually has a high pH, while soft water usually has a low pH. Aquarists in areas with very soft tap water should be especially aware of the buffering capacity of their tank, as this could lead to sudden and drastic swings in pH levels, potentially crashing the aquarium’s cycling process.

To counteract this issue, aquarists should increase carbonates to act as a buffer against sudden drops in pH or other extreme changes in water parameters. Monitoring your aquarium’s water chemistry and testing it regularly is key to having happy and healthy fish.

Methods of Fixing Hard Water in Fish Tank

There are several methods of fixing hard water in a fish tank, depending on the desired results. The two most common ways to reduce hardness levels are using reverse osmosis (RO) membranes or adding minerals.

Reverse Osmosis

Reverse Osmosis (RO) is an effective and efficient way to soften hard water, making it great for fish that need softer water. The RO system works by passing the water through a thin, permeable membrane with high pressure, which removes 95–99% of mineral compounds, hard metals, chemicals, and other contaminants and pollutants from the water.

Although this brings the pH and GH & KH levels in the output to 0—making it completely soft—it is not suitable for fish to live in as they need some minerals present in their environment. This can be achieved by combining untreated tap water or using a special salt mixture remineralized to balance out the minerals.

However, using an RO system comes with some drawbacks: It is slow and requires regular plans for producing soft water; the amount of treated water produced depends on the type of model installed; it’s quite expensive to install and maintain; maintaining an optimal pH and GH level also requires careful monitoring; and finally, the filter loses its efficiency over time requiring frequent replacement. Despite all these points – if you have a need for highly sensitive fish that only thrive in very soft water – reverse osmosis might be your best option.

Distilled Water

Distilled water is a great option for aquarists who need to lower their tank’s hardness level. Distilled water has a balanced pH with zero GH and KH, which means it will not affect the levels in your tank. However, it is important to note that distilled water also has very low dissolved oxygen levels, so you’ll need to aerate it before adding it to your aquarium.

Also, because you have to purchase large amounts of distilled water depending on the hardness level of your tank, consider using a remineralized to get the right combination of minerals and pollutants. With proper usage and maintenance, distilled water can provide an optimal environment for freshwater fish.

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Natural Rainwater

If you’re looking for an inexpensive solution to provide clean water for your aquarium, collecting rainwater may be a viable option. When collecting rainwater, it’s important to make sure that the air quality in the area is safe and clean and that you use sterile, food-grade containers. Avoid collecting the first batch of rain as pollutants and chemicals can be leached into the water.

Additionally, when collecting rainwater, keep in mind that it may not rain for weeks at a time so it’s important to have some stored. Mix collected water with tap water or aquarium salts to raise GH and KH levels. By following these tips, you can help ensure that your fish get clean and healthy water without breaking the bank.

Water Softening Pillows

Water softening pillows are chemical filtration media used to decrease the calcium and magnesium levels in the water. This is accomplished by replacing them with sodium chloride, reducing the water’s hardness. Saltwater aquariums may benefit from this method, but it is important to keep in mind that the added sodium levels might affect the fish’s tolerance.

The pillows usually fit inside the filter unit of an aquarium and can be recharged in a salt solution when they become less efficient. However, this method is not suitable for large aquariums since they can only treat a small amount of water at a time.

Peat Moss

Peat moss is a natural and effective way to lower the GH and KH levels of your fish tank’s water. The moss binds to calcium and magnesium ions, which are removed from the water, resulting in softer water with a lower pH and carbonate level. To use peat moss, you can put it in a mesh bag inside your filter for maximum effectiveness, or for shorter-term use, place it in a container or pillowcase in the tank itself.

Boil the peat moss and aerate the water after adding it to ensure it’s safe and effective for your fish. With time and observation, you will be able to adjust your tank’s parameters to perfect conditions for your fish.


Driftwood is a popular addition to aquariums. Not only does it look natural and provide a hiding spot for your fish, but it also releases tannic acid into the water which reduces the hardness of the water. It’s important to purchase from a reliable source, such as a pet store or online.

However, if you choose to find and clean your own driftwood, be sure to boil it for a couple of minutes or pour boiling water on it first to eliminate potential infections or hitchhikers.

Additionally, monitor the driftwood for any signs of algae or fungus growth once it’s in your tank. Tannins released by driftwood will turn the water brown unless you soak the wood for several weeks before adding it to your tank.


Macroalgae are a popular choice among aquarium hobbyists due to their ability to reduce nitrate, nitrite, phosphate, and ammonia levels in the tank.

Halimeda sometimes referred to as Money Plant is a type of green calcareous macroalgae which takes in calcium from the water for nourishment. It can act as algae in main tanks without needing a separate refugium.

While this macroalga is not often eaten by fish, it does go “sexual” after aging, which releases the absorbed minerals back into the water. Therefore, regular trimming of the older parts should be done to ensure the healthy and vibrant growth of this type of macroalgae.

Perform Frequent Water Changes

Macroalgae can efficiently absorb nitrate, nitrite, phosphate, and ammonia from your aquarium, but some species can uptake calcium from hard water. Halimeda is an example of a calcareous macroalga that absorbs calcium and builds it into its tissue as part of its support structure. It is non-invasive and many fish do not bother to consume it, making it suitable to put in your main tank.

Although this macroalgae will only slowly grow over time, it does go through a reproductive cycle which causes it to release all the calcium it’s absorbed back into the water. To prevent excessive uptake of calcium, make sure to regularly trim off older parts of the Halimeda plant.

Change Your Tank Water Source.

Using the right type of water for your tank is essential to maintaining a healthy aquarium environment. Rainwater is an inexpensive water source that can be used to lower the GH levels in a tank, but it should be collected from areas with clean air and stored in sterile, food-grade containers. It may be necessary to supplement rainwater with either tap water or aquarium salt in order to raise the GH level, as it is generally quite soft.

Knowing how different waters interact with tank chemistry is important when trying to alter your water’s hardness. Many factors must be carefully considered in order to achieve and maintain the ideal water conditions for your fish.

Add Water-Softening Crystals to Your Tank.

Softening crystals is a cheap and safe way to lower the hardness of tank water. These crystal mixes contain sodium ions which replace calcium and magnesium ions in the water, reducing their ability to bind to other ions.

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It’s important to read the instructions closely and use the correct amount of crystals: one liter typically softens 20 gallons of fresh water or half that for marine tanks. By dosing the tank water with softening crystals, you can help maintain an ion balance that is beneficial for your fish.

Reduce the pH of Your Water Source

Adding certain pH-reducing agents such as acid buffers and Indian almond leaves can help reduce the pH of your fish tank. Acid buffers, such as gallic acid, neutralize carbonate and bicarbonates to make the water softer, while Indian almond leaves contain tannic acid which gently removes minerals from the tank.

While it is important to keep the pH levels of your tank balanced for the well-being of your fish, do note that adding these agents may cause discoloration in the tank water. Observe how your fish react before making a decision and ensure their safety by keeping an eye on their behavior after introducing any changes into the aquarium.

Add Domestic Water Softeners

Softening fish tank water is possible with the use of domestic softeners, which are safe for this purpose. The machine softens water by replacing carbonate ions with sodium ions and reduces the GH and KH levels.

However, it is important to note that this process does not re-add minerals to the tank so you may want to remineralize the water. To use a softener in your fish tank, attach it according to the manufacturer’s instructions and then set the flow rate to a slow drip. This will ensure that the softened water is added gradually into the tank without shocking or stressing your fish.

Use Inert Lime-free Gravel.

Lime-free gravel is an excellent option for lowering hardness in fish tanks. Unlike gravels containing dolomite, limestone, or crushed coral, lime-free gravels do not accumulate magnesium and calcium ions which can raise the GH levels of your tank. In addition to being chemical-free, lime-free gravel is also safe for fish and other aquatic creatures.

Inert substances such as clay pellets, river rocks, and crushed granite can be used as lime-free gravel. It is important to remember that good maintenance practices are still necessary when using lime-free gravel. If left unchecked, debris such as fish waste and uneaten food can cause high GH levels in your tank. Therefore, regular cleaning and water checks need to be performed to ensure optimal water quality and a healthy habitat for your fish.

Remove Existing Tap Water Stains

Clean your tank with ease, using a calcium-lime-rust remover and 3% hydrogen peroxide to get rid of stubborn water stains!

Mix equal parts of water, CLR, and hydrogen peroxide in a container and apply the mixture to the build-up every five minutes for 30 minutes. This should help to break down any hard stains and reduce the overall GH levels in your tank.

Additionally, it is important to maintain proper water chemistry levels in order to prevent excessive calcium buildup. Regular water changes and filter maintenance can help ensure that your fish tank stays healthy and free of calcium buildup.


What is Water Hardness?

The hardness of water is determined by how much rock has dissolved in it, primarily limestone. Rain has an acidic nature and when it lands on or passes through limestone, it causes the softening of the rock, eventually leading to its erosion into bodies of water.

Experience the fascinating allure of limestone and dolomite, with their iconic mix of calcium carbonate and magnesium!

Will hard water kill fish?

Hard water can be detrimental to fish. PH levels can get dangerously high, preventing fish from performing vital functions necessary for their survival, potentially leading to death.

How can water hardness be reduced?

Adding soft water to your tank can help reduce hardness levels in the water. De-ionized water without minerals or rainwater can be purchased for this purpose. If the problem is severe, it may be necessary to empty the tank and replace all its contents. To start with a lesser degree, try replacing some of the contents and keep checking hardness levels until a healthy level is achieved.

Is hard water good for aquariums?

It is recommended to check the source of your water for high levels of minerals, such as calcium and magnesium, as these indicate hard water which is not suitable for aquariums.

What causes hardness in fish tank water?

Water in fish tanks can be hard due to the presence of minerals such as calcium and magnesium, which are commonly found in tap water.


Hard water is not recommended for use in aquariums due to the high levels of dangerous minerals. Though it may not always kill fish, its lack of essential trace elements and mineral imbalances can cause them long-term health problems. Using a water treatment product designed specifically for aquariums is a much better option if your tap water is hard due to the presence of these minerals.

By replacing your tank’s water regularly, testing it with a quality test kit, and keeping up with good maintenance practices you’ll be able to ensure that your fish stay healthy and safe.