When you move from beginner aquarium fish to more advanced species, water hardness becomes more important. Some species are very well adapted to the chemistry of their home waters and need it painstakingly replicated in the home aquarium. You must test for water hardness if you want them to thrive.
What Is Water Hardness?
Water hardness means different things to a chemist than it does an aquarium hobbyist. Technically, hardness is a measurement of how likely water is to leave mineral-rich residue as it evaporates. Functionally, hardness equals the concentration of calcium and magnesium in the water. This definition excludes other things dissolved in water, like organic compounds and sodium salts. Hardness is usually related to pH, but the relationship is not absolute. For aquarium purposes, hard water almost always has a high pH.
Types of Hardness
Test kits can measure two kinds of hardness. General hardness or GH is the most common. GH is more or less the same as a measure of total dissolved solids. For most fish, this is the kind of hardness you need to worry about. However, carbonate hardness or KH matters in some situations. KH measures the amount of carbonate minerals dissolved in the water. If you have certain aquarium plants or coral, you must monitor KH to make sure it's in the acceptable range. If you need to measure KH, you will need an additional test kit since most cannot measure both KH and GH.
You can test the hardness of your aquarium water with strips, also called dip tests. For these tests, you collect a sample of your water in a small receptacle. Never dip strips into the aquarium itself. The strip will change colors based on water hardness. You'll compare the strip's color to a chart that comes with the kit to determine the water's hardness.
Reagent tests work a lot like strip tests. For a reagent test you collect a small sample of the aquarium water in a test tube that comes with the test. Based on the manufacturer's instructions, you add a certain number of drops of a chemical -- the reagent -- to the water. The water will change color based on the water hardness. Like the strip test, the kit will come with a color chart for comparison. Discard the water after use; do not dump it back into the aquarium. Also, keep in mind that some reagents have expiration dates.
Measurements of Hardness
Both kinds of hardness and both kinds of test kits kick out results in two different units for their measurements. You can measure both types of hardness as ppm, or parts per million. This is typically written as ppmGH or ppmKH depending on the type of hardness. You can also measure hardness as degrees of hardness, written as dGH or dKH. Most sources will list an organism's needs in both formats, and many test kits' color charts have both units.