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What is Water Hardness?

02 March 2020


Understanding water hardness can feel daunting at first. But it is quite important to know about, because by simply researching the basics, you can greatly improve your ability to keep healthy and happy fish. Many fish can only thrive in certain levels of water hardness. So if the parameters are not correct, it can lead to stress and even death.

The hardness of water is dependant on the level of dissolved minerals found within the water. Where there is a large amount of dissolved minerals, the water will be hard, so of course a small quantity of dissolved minerals within the water will lead to the water being soft.

When water filters through permeable rock like limestone or chalk, it collects and carries minerals which causes it to become becomes hard. If your taps, pipes and kettles have limescale build up, you probably live in a hard water area. Hard water is common in areas like Lake Malawi, where the fish have adapted to a high pH and hardness. On the other side of the scale, soft water areas do not produce limescale and soap scum. Soft water is formed as water runs over impervious rock such as granite or filters through leaf litter, thus avoiding collectin many minerals at all. One example of a soft water environment is a jungle. Within jungle streams, fish such as tetras and corydoras adapt to the acidic water with a low level of dissolved minerals. Leaf litter and peat add tannic acid to the water, which then turns brown- this is known as blackwater.

The two types of water hardness are carbonate (also known as alkalinity) and total hardness. Carbonate hardness is mostly calcium carbonate (CaCO3). CaCO3 generally comes from chalk and limestone- it can be easily separated from the water by boiling it. A higher level of CaCO3 in water actually means the water has a great buffering capability (KH) and the pH will be more stable than water with lower KH. General hardness includes all minerals dissolved in the tap water. Boiling water will not remove general hardness.

Water hardness is measured in different ways. KH is measured in mg of CaCO3 per litre or ppm. General hardness is generally the kind of hardness which is referred to in fish care and is measured in mg CaCO3 per litre or ppm (despite other minerals which are present) or in dH. 1 dH = 17.9mg of CaCo3 per litre.

Monitoring the water hardness in the aquarium is very important as water that is too hard or soft for the fish can cause the immune system of the fish to reduce, leaving them susceptible to illnesses.

So how do you know if your water is too hard or too soft for your fish? You need to do your research. Most local aquatic shops are likely to use tap water to fill their tanks and on the whole, captive-bred tropical fish are hardy and adaptable. Some groups of fish are not so forgiving, though and Rift-lake Cichlids or Discus are examples of these more demanding specialist environments.

Testing the hardness of your tap water means you can easily determine the type of fish you should keep. For those looking to keep fish which are not suitable for the local available tap water, reverse osmosis water (RO water) may be your answer. RO water can free you from the tyranny of your unsuitable tap water completely. You can then use buffers to make the softest water suitable for even those Mexican natives that are used to swimming in liquid rock!