In some areas of the country, the water is extremely hard, meaning that there are a lot of minerals dissolved in it, most often calcium and magnesium. Hardness can be measured as German degrees of hardness (dGH), parts per million (ppm), or milligrams per liter (mg/L). Parts per million and milligrams per liter are analogous and are often used interchangeably in aquarists’ discussions. Most test kits typically measure calcium and magnesium, usually the largest mineral components to hard water.
Another measure is total dissolved solids (TDS), which quantifies all (or most of) the minerals dissolved in the water, no matter what they are. Electrical conductivity measures the water’s ability to transmit and electrical current. Most aquarists do not bother much with conductivity or even total dissolved solids, preferring to measure hardness by degrees, parts per million, or milligrams per liter.
Source: Sunken Gardens/ By: Karen A. Randall