Understanding GH versus KH
One of the questions asked every day in a fish store is to explain the
difference between GH (general hardness) and KH (carbonate hardness). Many
customers also fail to realize the distinction between hardness and high Ph
GH and KH are can be measured in
"degrees" or "parts per million (ppm)". Most test kits
available through retail outlets read in "degrees". One degree KH (dKH)
or GH (dGH) is equivalent to 17.8 ppm of Calcium Carbonate.
General hardness is usually what is being thought of when water is termed 'hard'
or 'soft'. If something prefers hard or soft water, it is GH which is important. GH does NOT
affect the pH but is the type of hardness that biologically affects organisms.
Calcium ions (Ca++) contribute
to (or are the main component of) general hardness, abbreviated GH.
KH measures carbonate and
bicarbonate ions and is the predominant component of alkalinity. KH is a
prime determinant of the pH in your water. High KH results in high pH. The carbonate buffering system
is the "natural" buffer for water.
How does this affect us as fish keepers? Well, in our area (and many others) the water supply
spends much time in contact with limestone.
These regions are said to have "hard water". Limestone is mostly calcium carbonate (CaCO3), which slowly dissolves
in water and dissociates into Ca++ and CO3--. This is the source of not only
those annoying hard water stains on sinks and faucets, but much aquarium trouble
In general, serious aquarists wishing to avoid hard water problems are advised
to turn to their water source for the solution. Simply put, if you are adding
large quantities of calcium carbonate into your water every time you perform a
water change and then wishing your hardness was lower, you have only yourself to
blame! Using 'pure' water sources, such as R/O (reverse osmosis) or DI
(de-ionized) water will completely eliminate the problem. Add proper minerals
and trace elements for the types of fish being kept to this source water and
your water will be perfect each time you add it!