Clarify what gH/kH is?

[holyherbiness]

I have been keeping aquariums for years but I have only recently heard of gH and kH (carbonate hardness or something?)

Is it absolutely vital to test for these things, because I usually only test for the basic ammonia/nitrate/nitrite/ph combo?

Someone explain to me what these terms are and what they mean.

 

[RTR]

GH refers to the concentration of calcium and magnesium ions in water, the ions which make it "hard" to form a foam with soap. We are not talking about laundry here, but those same ions have a negative effect on the eggs of blackwater fish, making it hard to breed those fish when those ions are present beyond certain levels.

KH is a measure of the buffering capacity of water, the ability of the water to resist change in pH from the addition of acids or bases. Nitrification is an acidifying process, so it is usually the ability to resist pH drops that we are most interested in. The buffer system in most natural water in the world is the carbonate/bicarbonate buffer system, so at best the reference to KH is talking about carbonate/bicarbonate, but in practice in tanks acids such as tannins and other buffers such as phosphates affect the measurement.