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Types of water hardness in the aquatic hobby - Measuring, knowledge & forum

Brief Description

The article below explains how to measure water hardness, describes the difference and relation between KH, dGH and pH. If you cannot find answer on your question on this page, feel free to ask us via form at the bottom of this article! Sharing experiences regarding water hardness in fish tanks is welcome too!

Introduction to water chemistry

One of the hardest subjects that most aquarists have trouble with is understanding water chemistry and what we are actually testing for in our water parameters. We all know why it is important to check ammonia, nitrites, and nitrates in our tank, but what is pH, KH, and GH. Hopefully from this article things will become clearer.

General hardness

Let’s start with General Hardness (GH) sometimes referred to as Total Hardness.

Depending on which type of land areas the water has passed through, varying levels of hardness are reached before it reaches its final destination i.e. if it passes over rocky areas it will have a higher mineral content, if it passes over marshy, forest areas it will normally create a soft water river or lake.

Fish like the neon tetras, and cardinal tetras are good examples of soft, acidic water lovers.

When we turn on our taps the mains water will contain many minerals that it has absorbed on its way to us, mostly magnesium & calcium, with small traces of other metals such as iron and zinc. The concentration of these minerals determines the general hardness of our water.

When we research which fish we are going to add to our tanks, the most favourable conditions given are usually represented by GH, pH, and Temp, as some fish prefer soft water like the south American cichlids, whereas the Malawi prefer really hard water, sometimes right at the top of the scale. To confuse matters worse most GH test kits will give their results in either DH (degrees of hardness) or PPM (parts per million) but don’t worry as there is a simple equation to enable us to convert DH to PPM or vice versa if either scale is preferred.

Below is a rough guide to the varying levels of water

DH PPM Water Hardness
0-3 0-54 soft
3-6 54-108 slightly soft
6-12 108-216 slightly hard
12-18 216-324 moderately hard
18-30 324-540 hard

These figures may vary slightly depending on which reference books you use. To convert from DH to PPM is simply a matter of multiplying the DH by 17.9 to give the PPM reading. Multiplying PPM by 0.056 will give you the reading in DH.

If we wish to raise the GH in our water it is simply a matter of adding calcium based rocks (limestone) or even coral gravel as these will very slowly dissolve into it, thereby increasing the mineral count. Some keepers will use calcium based powders to give a quicker end result but I prefer the slower process.

While it is possible for most fish to adapt to different GH levels, breeders do require getting a correct level in their tanks as it can affect fertility and hatch rates of eggs.

Carbonate hardness

Carbonate hardness (KH) is basically the buffering power of the water, which means how much acidity can be added to it before the pH is affected.

With an aquarium nitrates in the water will produce nitric acid, which in turn will reduce the pH if there isn’t enough buffering capacity to neutralize the effect. It is generally accepted that the KH should never drop below 4.5 else this can cause major pH instability which is detrimental to the health of your fish.

This is another reason why water changes are an essential part of fish keeping, to reduce the nitrates so that less nitric acid is produced, therefore a more stable pH is reached which will prevent it from dropping over a period of time. As expected the higher the hardness of the water the better the buffering capability.

In the simplest terms I can use KH is the concentration of bicarbonate & carbonate ions in the water. Carbonate ions will try to bond with hydrogen ions in the water, the more that bond the higher the pH, (the presence of free hydrogen ions acidify the water which will in turn lower the pH (power of hydrogen)).The result of this bonding is carbonic acid (H2CO3), which over a period of time will break itself down into carbon dioxide (CO2), this will then disperse itself into the atmosphere as oxygen will force it out of the water.

If the GH/KH of the water is too high for the fish that you wish to keep there are a couple of methods to reduce the levels. On my tanks I use RO (reverse osmosis) water, this removes a lot of the carbonate ions presents as well as the mineral content thus reducing the hardness of the water. The side effect of this is that the pH will then become more acidic which was fine when I kept my discus tanks, they were quite happy to live in water with a pH of 6 – 6.5 but I am having to remineralise my RO water now for the marine set up as my pH needs to be at least 8.2. This is why RO water is very unstable as the KH can no longer buffer the pH effectively. Adding the salt to the water brings the KH/pH balance back.

If the KH is too low Sodium Bicarbonate can be used to increase the KH (I found that adding 5ml to every 40 UK gallons of water would increase it by 4DH) and this did not affect my pH by too much, as I found out by monitoring it closely with the appropriate tests.

Adding oxygen to the water will drive off more CO2 thus increasing the buffering.

There are commercial buffers on the market that you can add to your water but check first that they will not increase your phosphate levels.

As you can see GH, KH, and pH are directly linked to each other to give the stability required for the water, if one drops it can affect the others. My best advice is that when buying fish, find ones that have requirements that match your water rather than adjusting the water to suit the fish. Adjusted water will try and readjust itself back to the original parameters over a course of time.

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