Indeed. Many aquarists distinguish two sorts of hardness, general and "carbonate" hardness (kH). General hardness is exactly what naegling23 described. "Carbonate" hardness is a good indicator of the buffering capacity. A high kH generall leads to a high pH, and also a fairly stable pH. As naegling pointed out hard water often has both a high gH and a high kH, but there are a number of salts that increase gH only, not kH.naegling23 said:Hardness does not necessarily equal buffering capacity, but gives a rough indication. Some salts, like calcium carbonate do provide buffering capacity (buffering is the ability of a solution to resist large changes in pH, think of it as a sort of sponge). Other salts like sodium chloride do not provide any buffering capacity. However, the more dissolved solids you have in the water, the more likely you are to have increased buffering capacity.