Description
This article is from the Beginning
Fishkeeping FAQ, by Thomas Narten with numerous contributions
by others.
35 General Hardness (GH)
General hardness (GH) refers to the dissolved concentration of
magnesium and calcium ions. When fish are said to prefer ``soft'' or
``hard'' water, it is GH (not KH) that is being referred to.
Note: GH, KH and pH form the Bermuda's Triangle of water chemistry.
Although the three properties are distinct, they all interact with
each other to varying degrees, making it difficult to adjust one
without impacting the other. That is one reason why beginning
aquarists are advised NOT to tamper with these parameters unless
absolutely necessary. As an example, ``hard'' water frequently often
comes from limestone aquifers. Limestone contains calcium carbonate,
which when dissolved in water increases both the GH (from calcium) and
KH (from carbonate) components. Increasing the KH component also
usually increases pH as well. Conceptually, the KH acts as a
``sponge'' absorbing the acid present in the water, raising the
water's pH.
Water hardness follows the following guidelines. The unit dH means
``degree hardness'', while ppm means ``parts per million'', which is
roughly equivalent to mg/L in water. 1 unit dH equals 17.8 ppm CaCO3.
Most test kits give the hardness in units of CaCO3; this means the
hardness is equivalent to that much CaCO3 in water but does not mean
it actually came from CaCO3.
General Hardness
0 - 4 dH, 0 - 70 ppm : very soft
4 - 8 dH, 70 - 140 ppm : soft
8 - 12 dH, 140 - 210 ppm : medium hard
12 - 18 dH, 210 - 320 ppm : fairly hard
18 - 30 dH, 320 - 530 ppm : hard
higher : liquid rock (Lake Malawi and Los Angeles, CA)
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