Aquarium Water Hardness
Hardness of water is related to the quantities of dissolved minerals it contains. In fish tanks the hardness level is important as each aquarium fish has an optimal range in which it should live. When hardness levels are low the aquarium is said to have soft water.
In a fish's natural habitat the water hardness is primarily determined by the geographical features of the catchment area in which it lives. Runoff containing limestone and other rock minerals will add hardness to the creek or lake in which it collects. Vegetated areas such as rainforests generally produce soft water creeks and rivers.
The water available to your home is also influenced in the same way. Domestic mains water is always harder than the water collected directly from falling rain. Once you know your local water hardness it will rarely vary in the future.
Some agents which contribute to hardness can be removed through reactions within the aquarium environment. Dissolved carbonates are often the cause. This carbonate hardness (KH) acts as a buffer on the water pH. As long as there is dissolved carbonate above 3 degrees of KH the pH will be stabilised in the alkaline range.
Even more important for aquarium fish is the permanent hardness value. This is usually linked to the calcium and magnesium levels within the water. It is also known as general hardness (GH).
This is the more important form of hardness because it is not fluctuating with any pH changes. When measuring and adjusting it should be permanent hardness that is concentrated on.
Measuring Water Hardness
Though all freshwater aquarium owners should monitor water hardness it
is rarely tested enough.The cost of an electronic general hardness meter has
become more affordable in recent years. Chemical hardness tests are the original
testing method and are also still available manufactured.
You can use a local resources to have your water hardness determined. City councils should have a good idea of what the tap water hardness should be in your area. Contact the water department and ask to speak with someone who tests the water for the best opinion. For specific hardness readings you can take a sample to your local pet shop who should be happy to test it for free.
There are numerous ways of quantifying the hardness level. In aquariums the degrees of hardness scale (DH) is the most commonly used but referring to it in parts per million is also useful for accurately applying adjustment dosage.
Adjusting Water Hardness
It is a lot easier to increase hardness than it is to reduce it. For this reason it can simplify matters if you choose fish suited to your local water conditions.
Manufacturing your own soft water can be done using a reverse osmosis filter but these are prohibitively expensive for the hobby fish keeper. Devices that remove hardness using ionic exchange can also be obtained.
Where practical, soft water fish are best kept in adjusted rainwater. This will not be possible in areas of significant air pollution however. Filtering tap water through peat should be used in such cases. Box filters can be packed with the dry material and the hardness will be removed as it passes through the filter. Demineralised water can also be used if required.
Increasing hardness can be achieved by using
products such as calcium carbonate, magnesium sulphate or calcium sulphate powders. When administering these products they may not immediately dissolve. A white sediment may settle on plants and ornaments but this will dissipate over a few days. Some clouding may also be experienced from the suspension of fine particles.
mg/l or ppm
0 - 3
0 - 54
3 - 6
54 - 108
6 - 12
108 - 216
12 - 18
216 - 324
18 - 30
324 - 540
To convert a DH measurement to the parts per million scale multiply the DH by 17.9. If you know your local water's hardness you can use this information to add the correct amount of calcium carbonate to your water.
The steps to follow are:
1. Determine your aquarium volume.
2. Subtract your Current DH from the goal DH to calculate the number of DH units needed.
3. Multiply this figure by 17.9 to convert it to ppm.
4. Multiply the ppm required by the aquarium volume to calculate the total mg required.
5. Divide the mg value by 1000 for the total number of calcium carbonate grams required.
Volume x ( [Current DH - Goal DH] x 17.9) divided by 1000 = grams required
For example if you have a 100 liter aquarium and your tap water is 10DH, and you wanted 15DH water it would take:
100 x ([15 -10] x 17.9) / 1000 = 8.95 grams
Decorative limestone rocks and coral substrates can also help to manage the water hardness. They will gradually dissolve over time rather than the sudden increase produced by powders. Using such materials in the aquarium is an excellent strategy where hard water fish are kept.
Related Aquarium Hardness Pages
Aquarium Water Topics
Aquarium Water | Aquarium Temperature |
Aquarium Heater |
Freshwater vs Marine |
Aquarium Calculators |
Aquarium PH |
Maturing An Aquarium