General Hardness

One of the easiest ways to increase water hardness is to add a small filter bag of aragonite or crushed coral (substrates marketed for marine aquariums) to your filter. The substrate will slowly dissolve, adding a steady source of both hardness and carbonate hardness (discussed later) to your water. You can also add these substances to your substrate, but hardness is harder to control that way. If the substances are added to the filter, you can always remove them or reduce the amount if you find that water is getting too hard. This method is best used in aquariums not receiving carbon dioxide (CO2) enrichment. The acidification of the water by CO2 and the rapid water flow through the filter may cause the aragonite to dissolve too quickly, increasing the hardness and the buffering capacity (alkalinity)...

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From a homeowner’s perspective, very hard water leaves a residue on silverware, glassware, and appliances; deposits in pipes and faucets; and can make it hard for soap to do its job in laundry. Many people in areas with very hard water install whole house water softeners. These devices typically replace calcium carbonate in the tap water with sodium chloride, although a few units use potassium chloride instead. The problem with replacing one molecule with a different one is that although the amount of calcium and magnesium may go down, the amount of sodium goes up by the same amount. Many plants can’t tolerate much sodium in the water, and fish that do better in soft water will have no easier time in sodium-filled water than they would in water with high levels of calcium. What is...

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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,...

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