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 (alkaline 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 as well.

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!