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 CaCO
3 - also called ppm in fish profiles
Fish profiles use either dH (German degrees or just degrees) or ppm (mg/l CaCO
3)
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.