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Water Hardness Units

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Offline Sue

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Water Hardness Units
« on: June 10, 2017, 11:41:05 AM »
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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











Offline MarquisMirage

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Re: Water Hardness Units
« Reply #1 on: June 12, 2017, 04:05:53 PM »
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German degrees can also be referred to as dKH (from the German 'Karbonathärte') as used in the API GH & KH test kits.  dGH (degrees General Hardness) can also be used for water hardness and is often used interchangeably with dH/dKH.

Offline Sue

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Re: Water Hardness Units
« Reply #2 on: June 12, 2017, 04:45:15 PM »
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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)

Offline Lynne W

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Re: Water Hardness Units
« Reply #3 on: January 15, 2018, 02:53:03 PM »
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Hi again, I've started looking at the seriously fish.com website to find compatible fish for my water which is 30.68ppm or 1.72dh.
The profile for Harlequins shows 18-215ppm so I assume they're OK for my water, yes?
But other profiles show "Hardness: 2 – 18°H" e.g. for Dwarf Gourami.
I initially thought they'd be out of range if using my 1.72dh, but then I found web page which showed a table which converted degrees of hardness of 0-3 = 0-50ppm.
So I'm now not sure how my 1.72dh relates to figures hardness when quoted as "2 – 18°H"?

Any help appreciated

Lynne 🐠

Offline Matt

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Re: Water Hardness Units
« Reply #4 on: January 15, 2018, 04:32:14 PM »
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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).

Offline Sue

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Re: Water Hardness Units
« Reply #5 on: January 15, 2018, 05:02:32 PM »
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Lynn's figures are for German degrees and ppm - she posted the full set of units from her water company in another post which gave the hardness in 4 units - 30.68 ppm; deg Clark 2.15; deg French 3.07 and deg German 1.72

I would round 1.72 up to 2 dH.

But I do need to warn you about dwarf gouramis. If they are imported from the far east there is a very good chance they'll be infected with the incurable dwarf gourami iridovirus. Even non-far east bred dwarfs could have come in contact with those from the far east and contracted it - at the importer or the wholesaler for example. I doubt either of these would keep fish of the same species from more than one source separate.
Unless you are really very keen on dwarf gouramis, I would avoid them.

Offline Lynne W

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Re: Water Hardness Units
« Reply #6 on: January 15, 2018, 06:30:26 PM »
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Thanks both, I'll avoid dwarf gouramis, thanks for the tip. The hunt for suitable fish continues 🐠🐠

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