gh & kh

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Active member
Dec 27, 2004
I recently picked up a GH and KH test kit. When I tested they were off the chart. What are acceptable levels and do I need to bring these down.



Well-known member
Jul 1, 2004
bowling green ky
In the search mode type in (toecutters) you should find a thread that if you follow boomers links you will be hooked up. If not P.M. me or ask anyone that is smarter than me to link it to the thread. HTH Steve


RF Staff
Dec 15, 2004
Duluth, Minnesota
GH = General hardness. It is the mesurement of water hardness. In includes Ca, Mg, Sr and Ba. It is often expressed as CaCO3, Calcium Carbonate. Most of these kits are for only freshwater as they don't go high enough for seawater. Since seawater is very hard it will be off the chart. Then kit can be used to give Calcium if it is capable to. Who's kit do you have.

KH = Carbonate Hardness, which is the same thing as Alk, Alkalinity. For reef tanks you want it to read 107 -225 ppm. Most often you hear of seawater, for reef tanks, where Alkalinity is 2..25 - 4.5 meq/ l. Natural seawater is 2.15-2.50 meq / l or 107 - 125 ppm KH

1 meq/ l = 50KH

107 ppm KH -->107 / 50 = 2.15 meq/ l
225 ppm KH --> 225 / 50 = 4.3 meq / l

A calculator for carbonate hardness, go to Convert Water Hardness Lets say the kit reads 125 KH and convert to meq/l

Type in 125, if that is what the kit reads in box and then select From ppm to Meq/ l = 2.5 meq / l


Also are you sure it is GH and KH or is it dKh and dGH, there is a big diffence there

Here is a calualtor GH but lets not go there yet until we know what you real have as a kit. An example would be you want 400 ppm caclium, type that in the first box and it = 1000 ppm CaCO3 ( the reading you would want from your kit)


I'm sure this will confuse you but here it is anyway.

Water Hardness

All fresh water sources contain calcium and magnesium in varying
quantities. These are cations with a +2 charge. They form salts with
anions which have a negative charge. The most important of these are
bicarbonate (HCO3(-1) ; carbonate CO3(-2) ; and sulfate

General Hardness (GH) measures the cations (+ charge); for calcium and

Carbonate Hardness (KH) Refers to only the bicarbonate, and carbonate
anions(-charge); it does not measures the sulfates and other anions.
Carbonate Hardness is a confusing term because it does refer to
hardness, but rather to the alkalinity (the ability of a solution to resist
a Ph change with an addition of an acid.) from the carbonates and
bicarbonates. Other anions (such as hydroxide, borates, silicates, and
phosphates) can contribute to the alkalinity. To be absolutely correct, you
should NEVER use the term 'kh'; however, this is often refered to in
aquarium literature. It should be noted that it is the
bicarbonate/carbonate buffering system which provides the majority of the
alkalinity in aquariums plant aquariums.

Kh and Gh are usually are close two each other, but the GH can be the same,
higher or lower to the KH depending on the Cations and Anions in the
sample. For example, a large amount of NaHCO3 would raise the (KH) and not
effect the (GH). A large amount of MgSO4 would raise the (GH) and not the

Usually, in fresh water most of the cations are calcium and magnesium (In a
3:1 ratio) and most of the anions are carbonates. The levels for (GH) and
(KH) will often be similar.


It would make sense to measure the general hardness in # of ions/liter or
molarity, but this is not used. The common units found in the literature
are degrees of general hardness dgh (GH) from the German system or ppm Ca
from CaCO3 . Carbonate hardness (KH) is a term which has nothing to d with
hardness, rather it is the amount of carbonate or bicarbonate equivalents
which effect the alklinity or acid buffering capacity. (KH) is equated to
ppm CO3 from CaCO3

Converting from dgh and dkh to ppm CaCO3 can be accomplished by multiplying
by 17.86

How the conversion factors were derived:
By definition 1dgh = 10mg/liter CaO
Atomic Weight Ca= 40 O=16 CaO= 56
So 10 mg/liter CaO contains 40/56 *10 = 7.143 mg/liter of Ca

By definition ppm Ca is not for elemental calcium but for ppm CaCO3 .
Atomic weight CaCO3=100
So 7.143 mg/liter of elemental Ca would be expressed as
100/40 * 7.143 = 17.8575 mg/liter(ppm)CaCO3.

1dgh = 17.86 ppm CaCO3 and 7.143 ppm Ca+2

1 dgh is defined as 10mg/lit CaO this can be related to ppm of CaCO3 as in
above. Now the definition for dKh must have come from the amount of
carbonate in 17.86 ppm CaCO3 which has nothing to do with GH wich is
defined by CaO!
Historically GH must have been defined first in terms of CaO; hardness in
terms of ppm CaCO3 second, then KH third?

1dKh = 17.86 ppm CaCO3

>From above; 1dkh = 17.8575 mg/liter CaCO3. 7.143 mg/liter of this is Ca,
the rest ;(17.8575-7.143)= 10.7145mg/liter CO3

1dKh = 10.7145 ppm CO3

For bicarbonate:

CaCO3 forms Ca(HCO3)2 in water at ph less than 10.25 . (Two bicarbonates
are formed from each carbonate ion):

CaCO3 + H20 + CO2 ---> Ca(HCO3)2

CO3 mw = 60
HCO3 mw = 61

10.7145mg/liter CO3 from CaCO3 (each CO3 carbonate anion forms two HCO3
bicarbonate anions; 61/60*2 *10.7=21.8 mg/liter HCO3

Another way to calculate this is using molarity:

1dkh = 17.86 mg/liter CaCO3
mw CaCO3 =100
17.86 mg/liter CaCO3 = .179 m Mole CaCO3

This will form 2* .179 m Mole = .358 m Mole
Multipling moles *mw will give mg:
0.358*61(mwHCO3) = 21.8 mg/liter HCO3

1dkh= 21.8 ppm HCO3

How to use these conversion factors:

If you have alkalinity in ppm or hardness in ppm divide by 17.86 to get degrees.

If you want to raise the alkalinity by 1dkh using CaCO3:
use 17.86 mg CaCO3
If you want to raise the alkalinity by 1dkh using NaHCO3:
mw Na =23
mw HCO3= 61
mwNaHCO3= 84
1dkh= 21.8 ppm HCO3
21.8 *84/61=30 mg/liter of NaHCO3

using molarity:
0.358 mMoles * 84(mwNaHCO3) = 30 mg/liter of NaHCO3
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