How does the hardness of the water in the aquarium affect your fish?

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Ammonia, check. Nitrite, check. Check nitrate. Water hardness, right?

Water hardness is an important water parameter for healthy saltwater and freshwater fish tanks, but most hobbyists don’t test for it and don’t even know they exist! Knowing the hardness of water in your aquarium and understanding what affects it and how it is affected gives you more control and insight into your entire ecosystem.

Read on to find out what water hardness is, how it affects your tank, and how to test and control it in your own freshwater aquarium!

What is water hardness?

General water hardness, sometimes called total hardness, is important as it is related to the pH of the water and acts as a kind of buffer. If you have a problem with your pH and need to either increase or decrease it, the first thing to do is adjust the hardness. The general water hardness should not be confused with the carbonate hardness.

In terms of aquarium terminology, general hardness measures the number of minerals in the water, namely calcium and magnesium ions. These are important minerals for many fish and invertebrates in the aquarium as they help build bones and exoskeleton, regulate metabolism, and facilitate ion exchange. If fish are kept outside their optimal hardness range, they may not grow as big or live as long.

In general, the hardness varies depending on the geographic location. Local freshwater reservoirs can be affected by geological composition, runoff, seasonal weather conditions and other external factors. Because of these external factors, tap water is usually moderately hard to hard and needs to be softened for certain freshwater aquarium fish.

How do you measure the water hardness in your aquarium?

The general hardness is usually abbreviated as GH and measured in degrees. it is common to see it notated as dGH or ° GH. The general hardness can also be documented in ppm (parts per million), whereby 1 ° GH corresponds to 17.848 ppm. There are many commercial aquarium test kits that can be used to accurately measure both general hardness and carbonate hardness.

Carbonate hardness (KH; also known as alkalinity) measures the carbonates and bicarbonates in water, which are usually recorded as dKH or ° KH.

Will hard water kill your aquarium fish?

As with any water parameter, if the overall hardness is outside of the range intended for your fish, you are most likely to have problems in your aquarium. It is important to understand the natural aquatic conditions under which fish thrive so that their metabolism and physiological processes function as they should without using too much energy.

Certain types of fish occur in certain degrees of general hardness:

Very soft water (0-3 ° GH / 0-50 ppm). At the extreme end of the general hardness range, very soft water can be difficult to lift. You are unlikely to find many freshwater aquarium fish that prefer this area, if any. One notable species that fits into this area is the Killifish / Banded Panchax (Epiplatys annulatus) clown.

Soft water (3-6 ° GH / 50-100 ppm). Most of the aquarium fish in this category come from South America, where the water is naturally filled with tannins from rotting plants and other debris. Some popular fish are tetras, angelfish, corydoras, and South American cichlids.

Moderately hard water (6-12 ° GH / 100-200 ppm). This area of ​​general hardness is preferred by most species of tropical fish. This includes most livebearers such as mollies and platies and is the optimal range for Betta fish.

Hard water (12-18 ° GH / 200-300 ppm). In general, most types of aquariums with moderately hard water, such as platies and mollies, can withstand slightly harder water as long as conditions remain stable. At this point, however, you should consider lowering the overall hardness or keeping African fish species that can handle higher grades, such as congo tetras (Phenacogrammus interruptus) and African cichlids. Odessa barbs (Pethia padamya), flag fish (Jordanella floridae), and some species of rainbow fish (family Melanotaeniidae) are also known to appreciate hard water.

Very hard water (18+ ° GH / 300 + ppm). This area is sometimes referred to as the liquid rock zone in the aquarium hobby because there are so many minerals in the water. While most fish can adapt to higher or lower degrees of hardness, it is always best to adapt the fish species to the parameters of the tank. Most African cichlids also thrive in very harsh water conditions, along with some cichlids from Central America.

It is important to note that these ranges are general estimates and different hobbyists may have wider or narrower ranges for each category.

Which fish do well in hard water?

Most freshwater livebearers can adapt to hard water. However, once the water is classified as very hard, your stocking will be mostly limited to African cichlids that are endemic to a particular lake in the world. This is because these lakes, like Lake Malawi and Lake Victoria, have rocky bottoms that keep the hardness high. Some Central American cichlids are also better suited to hard water than soft water aquariums.

It should also be noted that most living aquatic plants can withstand a wide range of general hardness; However, some hobbyists have found that certain types of algae grow better in aquariums with more extreme ranges and that aquatic plants are overwhelmed.

How do you increase the water hardness in your aquarium?

While there are “ideal” parameters that make for perfect water chemistry, most tanks run outside of these ranges. In general, it is better to have stable values ​​than constantly changing attempts to reach those values. This can quickly unbalance your tank and lead to stress on your animals.

The same principle applies to manipulating the total water hardness in your aquarium. You have to keep in mind that you can also accidentally change the calcium and magnesium values, the pH value and the carbonate hardness.

However, if your tap water is softer water or you want to keep freshwater fish that need a little more specific water chemistry, then you need to know how to increase the mineral levels in your aquarium.

Crushed coral and limestone

Adding crushed coral to an area with high water flow, such as the aquarium filter, is a popular way to increase both general hardness and carbonate hardness. However, it can be tricky figuring out the exact amount it will take to get your tank to the level you want.

Similarly, limestone can be added to the main aquarium display or filter to increase both KH and GH. Both crushed coral and limestone release minerals into the water over time, increasing concentrations. this also leads to an increase in pH. The speed at which the crushed corals and limestones dissolve in your tank depends largely on how acidic the water quality is.

Other aquarium safe products

Some hobbyists use Epsom salts to add hardness to their aquarium because it contains magnesium. However, it does not contain calcium and these values ​​would otherwise have to be increased. We don’t recommend using Epsom salts as it can be difficult to dose and can have permanent effects on fish and invertebrates.

If you can’t use crushed coral or limestone, it may be best to use aquarium-safe calcium / magnesium products that are specifically designed to increase overall hardness. Follow the directions and do a water test regularly to safely track general hardness, carbonate hardness, and pH.

If you are having trouble maintaining these levels in your tank due to the spring water you are using, it may be best to switch to distilled water or reverse osmosis water to have more control over these variables.


Overall hardness can definitely be a confusing aspect of aquarium water quality, but it is critical to maintaining a stable and safe environment for your fish. Different fish require different total hardnesses depending on where they come from in the world. These values ​​need to be adjusted in your aquarium by adjusting them as needed. Regular general hardness, carbonate hardness, and pH tests must be performed to determine how these parameters affect each other.

Always remember that while there are ideal parameters for freshwater aquariums, it is almost always better to have a stable tank with unideal numbers than to chase after them and disrupt your entire aquarium system.

If you have any questions about the general hardness, carbonate hardness (alkalinity) or have experience with a fluctuating pH value in the aquarium, please do not hesitate to leave a comment below!

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