Understanding water hardness


Water hardness — not the most exciting of subjects to try and fully understand. The extensive amount of terminology makes the subject seem endless and confusing in equal measure.

Unfortunately, it is also one of the most critical elements of water chemistry and has a big effect on both our beneficial bacteria, and on the fish themselves. Is hardness a bit too hard to understand? Let’s see if it can be softened up...

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What is Water Hardness?

Put at its simplest, hardness is a measure of how much dissolved rock there is in your water; specifically, the amount of dissolved limestone it contains.

Rain is acidic, and when it falls on, or soaks through, limestone it creates a softer rock that is washed away into the water. The two main rocks are limestone, a sedimentary rock composed primarily of calcium carbonate, and dolomite, which is calcium carbonate with magnesium.

Do you have hard or soft water?

There is a general Southeast/Northwest divide, caused by the underlying rocks in these areas. Cumbria, Lancashire and Cheshire have some of the softest water in England, and huge areas of Scotland have water so soft that it is nearly pure H2O. Conversely, Kent, Essex and Cambridgeshire have extremely hard water. Assumptions should never be made regarding water quality, but take a look at your local water supplier’s website for a free quality report. It should provide all of the necessary information.

The next step is to get a KH and GH test kit. Most of these work by adding drops of the test liquid to a sample of water, and counting the number of drops until there is a distinct colour change (a technique known as titration). This provides your hardness level.


GH refers to ‘general hardness’ and is a measure of the calcium in the water.  Fish, like humans, need calcium for strong bone development, for sending signals to their muscles, and to keep their hearts beating. Calcium absorption in fish is via their gills.  Soft water fish have evolved to take in as much as they can, whilst hard water fish require more calcium to be present.


KH is a measure of the carbonates and bicarbonates. These minerals are a crucial presence because they act as buffers and maintain a stable and healthy environment for the fish.

Think of KH like little sponges, soaking up the acids produced in your tank. Most of these acids come from different stages of the nitrogen cycle. As ammonia becomes nitrite it releases a hydrogen (H+) ion which, if not soaked up, will lower the pH. Nitrates at the end of the cycle can form nitric acid and, again, need to be soaked up by the KH buffers.

For those aquarists who use carbon dioxide (such as plant-growing aquascapers), KH buffers the tank as the carbon dioxide forms carbonic acid. That is why it’s crucial to monitor your carbon dioxide and hardness. The softer the water, the more careful you need to be when injecting carbon dioxide.

Does it really matter if I have hard or soft water fish? 

Yes, it does. Hardness and pH are linked, which usually means that the pH is more acidic in soft waters and more alkaline in hard waters.

Fish that have evolved in soft acidic water and those that have evolved in hard alkaline environments have totally different mechanisms for maintaining healthy levels of minerals, such as sodium and calcium. When a hard water fish is placed into soft water, it is unable to absorb sufficient minerals, and these cannot often be ingested through diet alone. This is why some species of Molly in soft water will develop shimmies, twitching and muscle spasms, often shortly before death.

Conversely, placing a soft water fish into hard water may result in them taking up too much of one mineral at the expense of others. This can lead to deficiencies and organ problems. The effects of this will show gradually over time.

Soft water

There’s a wide range of fish from the Amazon basin which are ideal for soft water environments, including species such as tetras, plecs, dwarf cichlids and Corydoras.

The issue with soft water is, that if the tank is overstocked, or the water is not regularly changed, all the buffers will be used up. The little sponges will become saturated, gradually lowering the pH. Eventually, it drops too low for the filter bacteria to cope with and they die off.

Nothing will appear to be wrong immediately because the ammonia that could kill the fish is locked away as ammonium.  Whilst this is not as imminently hazardous, it is a ticking bomb waiting to go off. If the water is changed, there will be an increase in the pH and, almost instantly, this causes the ammonium to return to its state as toxic ammonia. This is why some people believe that water changes are bad for the tank.

Soft water requires smaller, but more frequent water changes, and the addition of buffers should be considered in order to slightly raise the hardness.

Hard Water

Unless you have water so hard that it’s akin to ‘liquid rock’ coming out of your tap, hardwater fish are almost as plentiful. Fish from the Rift Lakes in Africa thrive in mineral-rich conditions. From Mbuna to shell-dwellers, and some species of Synodontis, they will all benefit from the calcium levels and stable waters of a hardwater tank.

The aquarium staples of Guppies, Platies, Swordtails, and Mollies need harder water than most fish to survive, and will often suffer in soft water. Rainbows, and many of the gourami species, will do well in moderately hard to hard water. For a temperate tank, consider Rainbow shiners, Paradise fish, and, if there is enough, flow Hillstream loaches.

Choosing the right fish for the right water simply takes some research and consideration, but there are plenty of options available.

The only danger here is that as water evaporates only pure water leaves the tank. When you top up you add more GH and KH, and over time this makes your water harder.
If you have to top up use pure water (such as RO water) to offset the evaporation.

Changing Hardness

Sometimes there is the desire to keep fish which are unsuitable for your water. It can be tempting to add them and wish for the best, but it’s unfair to have fish suffer as a result.

Avoid just use pH altering products. The fish have adapted to their environment and it is necessary to consider their conditions as a whole, not just the addition of extra ions.

Increasing KH and GH

Increasing the hardness is relatively easy and there are numerous products and buffers available which mimic natural limestone.

Lowering KH and GH

Reducing from very hard water to soft water is a more difficult task. Whilst adding leaves and bog wood can slightly lower the KH and reduce the pH, this is not highly effective, especially as each water change dilutes the effect.

Instead, consider using reverse osmosis water as this is stripped of all minerals and carbonates. Adding buffers back in to mineralise the water to the desired hardness is also an idea and it is also possible to mix tap water in to rebalance the necessary levels.

Hardness may not be the most interesting of topics, but it is a crucial element of fish health, and for keeping your tank in good condition. It is certainly a lot more satisfying to keep fish without any issues and, by choosing the right fish and synchronising it with your water and tank conditions, things are a lot less difficult.