Water hardness is also known as general hardness. The test kit would be labeld 'GH'. To put it very basically (I'm no chemist) it is a way of measuring the amount of dissolved elements (salts, minerals etc.) in the water. A low PH tends to be accompanied by a low GH, while a high PH tends to be accompanied by a high GH, though this isn't always the case. General hardness is more of a concern for laundry then for fish, but there is no harm in testing it to make sure it is at acceptable levels (acceptable is actually a very wide range for most fish).
