IceH2O said:
Some of you are confusing KH and GH. Hardness is GH not KH.
KH has nothing to do with how hard the water is. It has everything to do with pH buffering though so you are right that you need some, 3 is the lowest that is considered safe.
If water is soft its because it has a low GH. Its acidic because it has a low KH which in turn means a low pH.
GH- general hardness ( Ca/Mg )
KH- alkalinity
Well, yes & no. KH is also call carbonate hardness, and it does have something to do with GH.
For most people, high GH is associated with a high KH. The Ca/Mg in the water must be associated with an anion, and for most it is CO3 or HCO3 (ie KH). It is possible to have high GH & low KH (eg having CaCl2 in the water) but that is uncommon.
All of this is academic, however. For the fishies, pH stability is key. For that, you need adaquate buffers. You have to have either adaquate KH, or an added buffer of your choice. The actual GH or KH number really is not that important.