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Water Hardness Numbers

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The hardness or softness of water is often a a source of confusion to aquarium owners. While entire volumes can be written about the chemistry behind water hardness, just understanding how the water hardness numbers equate to words such as soft or hard is what many aquarists want to know. When researching habitat needs for a fish, owners may be told to keep the fish in a specific type of water, such as medium hard. But what does medium hard mean in actual numbers? In other cases the aquarist may be told the water should be 20-40 mg/l of hardness. Great, now the aquarium owner has a number to work with. Yet when testing is performed on the water, the test kit shows results as dGH. Is dGH the same thing as mg/l?

Units of Measurement
Units most commonly used to represent hardness in aquarium settings, are dGH, dH, mg/l, and ppm. Other units sometimes used, but less commonly seen are gpg, mmol/L, Clark degrees (an English measurement), and f degrees (a French measurement). While it's not critical to know what those acronyms mean, it's sometimes helpful to know they are, so I've included a key below. The important thing to understand is that they are not all exactly the same. That means it's wise to know what units of measurement your kit uses, so you can determine what those numbers mean in relation to soft or hard water.

dGH - stands for degrees of general hardness or GH
dH - stands for degrees of hardness, and is an abbreviation of the German term, Deutsche Härte
mg/l - stands for milligrams per liter
ppm - stands for parts per million
gpg - stands for grains per gallon
mmol/L - stands for millimoles per liter
°Clark - named after British chemist, Thomas Clark, same as °e.
°e - stands for English degrees, which is the same as Clark degrees
°f - stands for French degrees

What About KH?
You may have noticed that KH is absent from the list above. That is because it isn't the measure of the water hardness used when referring to fish habit. When aquarium articles refer to hardness levels for fish, they are describing general hardness (GH). KH is sometimes referred to as temporary hardness, or carbonate hardness, but more a more accurate term for it is carbonate alkalinity. KH is more important in relation to water pH, while GH is the measure used when referring to the softness or hardness of aquarium water.

Specifically KH is the meaure of the carbonate and bicarbonate anions, while general hardness (GH) is the measure of calcium and magnesium cations in the water. Be aware that although KH and GH values may be close together, they are not referring to the the same thing. As previously stated, most literature about fish habitat is referring to the general hardness (GH), rather than the KH, when stating that water is soft or hard. Water test kits may test for either GH or KH, or in some cases, tests for both are included. So pay close attention to what the product indicates before purchasing.

What Water Hardness Numbers Mean
Now comes the million dollar question, after testing the water, but how does your test kit number equate to something that is useful in terms of water hardness? The chart below shows water hardness numbers in the most commonly used units, split by very soft to very hard water. A word of advice, if your water source is quite different than the needs of your fish, you might want to consider choosing different fish. Another option is to find a different water source. Either way, be aware that even though there are products for changing the hardness of the water, it can be challenging to maintain it at the desired level.

Water Hardness Levels

Hardness Level dGH mg/l ppm
Very Soft 0 - 4° dGH 0 mg/l 0 - 60 ppm
Soft 4 - 8° dGH 1 - 20 mg/l 60 - 120 ppm
Medium Hard 8-12° dGH 20 - 50 mg/l 120 - 180 ppm
Fairly Hard 12-18° dGH 50 - 120 mg/l 180 - 240 ppm
Hard 18-30° dGH 120 - 175 mg/l 240 - 500 ppm
Very Hard >30° dGH >175mg/l >500 ppm
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