Testing for KH – Carbonate Hardness 101

Carbonate Hardness is something that you need to understand and to take in consideration when you think of your water. Although it does not directly affect your shrimp or fish, it does play a big part in your waters pH (Potential Hydrogen/Alkaline or acid level) in your tank.


The “K” in kH is from the German word Karbonate and is the measure of Bicarbonate and Carbonate ions in your water. These particular ions will act as a protective shield or reinforcement to large pH shifts in your water. pH stability is vital to your tank mates health for many reasons (Learn more about pH here: Potential Hydrogen-comingsoon). kH is measured in degrees; specifically 1 degree of kH equals 17.9mg per Liter or ppm  (parts per million). In simple terms the higher number of degrees, the more concentrated or potent your water is with kH.

Carbonate Hardness and General Hardness seem similar but are unrelated. General Hardness is a measure of Calcium and Magnesium and other dissolved minerals in your tank. It is a common misunderstanding that the two are connected – that your kH and gH levels will be similar to each other or your kH can never be higher than gH. However, there are many cases where in certain areas of the world you can find areas where the kH is naturally higher than gH. Also water treated with water softeners can have a high kH with a low amount of gH.

You will need to consider your type of water, plants, type of fish and shrimp, and the level of pH you need to maintain to determine what level of kH your tank should be at.

In a planted tank pH levels can fluctuate regularly if there is no kH present in your water.  When sunlight is not present (at night or when your light is scheduled to be off) the plants in your tank stop taking in CO2 and producing oxygen, everything else that is present in your tank that produce CO2 will still continue to produce it. This co2 production will attach to your water molecules and produce Carbonic acid. The more Carbonic acid present in your water the lower your pH will become. Once you turn your light on or the sun comes up, your plants will start producing oxygen and taking in carbon dioxide again and your ph will raise. This pH rollercoaster or “pH Shock” is very stressful and can kill any living creatures in your tank.  Higher kH levels will act as a barrier or buffer and will prevent this dip in pH from happening.


However if you do not have many plants in your aquarium and you have creatures in your tank that need a more acidic environment a High kH can actually be trouble. If you have a high kH it will make it extremely difficult if you need to lower your pH to a level that your fish or shrimp can be happy with. So in this case it is a good idea to keep your kH levels as close as possible to zero because you wont be able to lower your pH without lowering your kH.

kH levels can fluctuate on their own in your tank for different reasons. If you add water into your tank with a higher kh level naturally your tanks kH concentration will rise – and if you add water with low or zero kh naturally the concentration of kH in your water will lessen. kH is also consumed by Nitrifying Bacteria which is a natural part of the nitrogen cycle in your tank and will lessen your kH (as well as pH) over time.  Certain substrates can generate kH in your tank to keep it at a constant level (good if you need it but bad if you don’t), however after a very long period of time its effectiveness can dwindle. Water agitation in your tank can also break down the carbonates in your water, so if you have a strong filter and strong current running you will have a lower amount of kH.

If you are in need to change the level of kH in your water:

In order to raise kH you can use several metods but the easiest and cheapest would be to use pure baking soda (Sodium BiCarbonate), it will not change your gH but will raise your pH because of the hydrogen. 1 level teaspoon (15ml) dissolved in 50L (13 gallons) of water will raise the kH by 4 degrees (naturally 1/4th of a teaspoon will raise 50L by 1 degree). I suggest on using this method not on your tank itself but just on water you will be using for water changes. This way you can test the water for its kH level before you put it into the tank as a precaution. Also it is never a good idea to drastically change any water parameter in one go.  Gently raising your pH and kH every day if needed will be easier on your tank mates. Think of it like adding salt to something you are cooking – You can easily add a bit more, but its very difficult to take away if you put too much. (what I mean here is that if your constantly drastically changing the water parameters from too low and too high to get what you want it will be extremely hard on your livestock and they will suffer)

If you have a natural High kH in the water  using reverse osmosis water or clean rain water which should have 0 dkH will work with diluting the amount of kH in your tank. Also, if you use as a source (tap water) for your water changes, boiling your water beforehand will destroy the carbonates in that water. Make sure you let it cool down after and that it is at the same temperature as your tank water before you add it into your tank. Remember to do it gradually, drastic water changes will stress out livestock.

I hope you have a general understanding on how kH levels can be important to your tank.