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. The hardness of the water has a huge effect on both our beneficial bacteria, and on the fish themselves. Hardness can be hard to understand, however I will try to soften it 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 dissolves into the water. This process usually involves two main rock types. Limestone, a sedimentary rock composed primarily of calcium carbonate, and dolomite, which is calcium carbonate with magnesium.
Do you have hard or soft water?
Ageneral Southeast/Northwest divide exists, caused by the underlying rocks in these areas. Cumbria, Lancashire and Cheshire have some of the softest water in England. Also, huge areas of Scotland have water so soft that it is nearly pure H2O.
Conversely, Kent, Essex and Cambridgeshire have extremely hard water. You should never assume the water quality that comes out of your tap. You can look at your local water supplier’s website for a free quality report. It should provide most of the necessary information, however, it is always worth double checking your tap water.
The first step to check your water 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. You need to count the number of drops until there is a distinct colour change (a technique known as titration). This provides your hardness level.
Related article: What you need to know about using a test kit.
GH
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
KH is a measure of the carbonates and bicarbonates. These minerals are crucial because they act as buffers and maintain a stable and healthy environment for the fish.
Think of KH being 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, 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 for your aquascape.
Does it actually matter if I have hard or soft water fish?
Yes, it does. A relationship between hardness and pH exists. Usually, the pH is more acidic in soft waters and more alkaline in hard waters.
Fish from soft water and those from hard environments have different mechanisms for maintaining healthy levels of minerals. If you place a hard water fish into soft water, it is unable to absorb sufficient minerals. These minerals cannot solely come from diet alone, and a lack of them can be fatal. Sometimes, you will see a molly in soft water develop shimmies, twitching and muscle spasms, often shortly before death.
Conversely, placing a soft water fish into hard water will also cause problems. They can end up with 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
A wide range of tropical fish from the Amazon basin are ideal for soft water environments. This includes species such as tetras, plecs, dwarf cichlids and cory catfish.
The issue with soft water is the lack of buffering capacity. The buffers can quickly run out if the tank has too many fish or water changes are not frequent enough. 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, the ammonia that could kill the fish is present as ammonium. Whilst this is not as imminently hazardous, it is a ticking bomb waiting to go off.
After a water change, there will be an increase in pH. Almost instantly, the ammonium will return to the 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. The addition of buffers can be beneficial 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. Mbuna, shell-dwellers, and some species of Synodontis all appreciate the calcium and stable waters provided by hard water.
The aquarium staples of guppies, platies, swordtails, and mollies need harder water than most fish to survive. They 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. Plenty of options are available. You can find some suggestions across our website or visit your local store for advice.
The only danger here is that as water evaporates only pure water leaves the tank. When you replace this with tap water 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), as this lacks the dissolved minerals.
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. Instead, you should consider their conditions as a whole, not just the pH.
Increasing KH and GH
Increasing the hardness is relatively easy and there are numerous products and buffers available which mimic natural limestone. These can increase the calcium levels, carbonate levels, or both.
Lowering KH and GH
Reducing from hard water to soft water is a more difficult task. Whilst adding leaves and bogwood can slightly lower the KH and reduce the pH, this is not highly effective. Especially since each water change dilutes the effect.
Instead, consider using reverse osmosis water as the semi permeable membrane removes the minerals and carbonates. Adding buffers back in to remineralise the water to the desired hardness is possible. Equally, it is 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. Choosing the right fish and synchronising it with your water and tank conditions, means things are a lot less difficult.
Related article: Frequently asked questions on lowering hardness