I know about ph, and temperature, but what does it mean by the water being a certain hardness?
Hardness is defined as the concentrations of calcium and magnesium ions expressed in terms of calcium carbonate.
Most natural water supplies contain at least some hardness due to dissolved calcium and magnesium salts. Other minerals, such as iron, may contribute to the hardness of water, but in natural water, they are generally present in insignificant quantities.
Water hardness has a major effect on pH and pH stability. It will affect the toxicity of many common substances, including some fish disease treatments.
Different species of fish have varied water hardness requirements, so it is important to find out what hardness is best for your fish.