Water Hardness Units

There are several units used for water hardness. The problem for fish keepers is that water companies rarely use the same units as fish profiles.

The units I have come across are:

German degrees - also called dH in fish profiles.
Clarke degrees
French degrees
mg/l Ca
mg/l CaO
mg/l CaCO₃ - also called ppm in fish profiles

Fish profiles use either dH (German degrees or just degrees) or ppm (mg/l CaCO₃)

It is quite easy to convert one unit into another. The Fish Calculator section in the menu at the top of the page converts between German degrees and ppm. But for other conversions you will need a calculator (or pen and paper if you are good at arithmetic  )



To covert hardness given as degrees Clarke:
1 deg Clarke = 14.28 ppm or 0.8 dH

To convert hardness given as mg/l Ca:
1 mg/l Ca = 2.5 ppm or 0.142 dH

To convert hardness given as mg/l CaO:
1 mg/l CaO = 1.78 ppm or 0.1 dH




Most UK water companies give our hardness in numbers, and we should also make a note of the unit of measurement.
Three water companies do not give numbers. At the time of typing, these are Northumbrian Water, Essex & Suffolk Water and Welsh Water/Dwr Cymru. They give the hardness in words. This can be misleading as the words usually imply that the water is harder than we as fish keepers consider it to be.


Water company bands are:

Soft:
0 to 2.8 dH and 0 to 50 ppm. We would call this very soft.
Moderately soft:
2.8 to 5.6 dH and 50 to 100 ppm.We would call this soft.
Slightly hard:
5.6 to 8.4 dH and 100 to 150 ppm. We would call this soft.
Moderately hard;
8.4 to 11.2 dH and 150 to 200 ppm. We would call this top end of soft to middling.
Hard:
11.2 to 16.8 and 200 to 300 ppm. We would call this middling to hard.
Very hard:
Over 16.8 and over 300 ppm. We would call this very hard.

GH German deg should not be interchangeable with KH German deg. My GH and KH have different numbers of German degrees. (5 for GH and 3 for KH).

GH measures just divalent metal ions which is mainly calcium with some magnesium and trace amounts of other metal ions.
KH measures buffering chemicals, mainly bicarbonate and carbonate.

Water companies use words like slightly hard, moderately soft etc, but humans want numbers. It would look very complicated if they said moderately hard = this much calcium, this much magnesium, this much x, this much y, this much z. So they work out what the mg/l would be if it was all Ca or all CaO or all caCO₃.
Mg/l  or ppm CaCo₃ doesn't mean the water contains this many ppm calcium and the same number ppm carbonate; it means that if all the divalent metal ions were added together then treated as if it was all CaCo₃, then this is what the ppm would be.

It took me ages to get my head round that concept!


KH and alkalinity are interchangeable. KH is used by fishkeepers while water companies use alkalinity for the same thing. Alkalinity is measured by taking a set volume of water and adding a specific acid at a set concentration until the pH drops to 4.5. The amount of acid it takes is the basis for the alkalinity value. (Info from son)

Sue is better versed in this stuff but I think this is referring to Clarke degrees, also known as English degrees on some water company websites (United Utilities for one).

Thanks both, I'll avoid dwarf gouramis, thanks for the tip. The hunt for suitable fish continues 🐠🐠