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Water hardness is defined by the amount of calcium and magnesium compounds dissolved in a water supply. Measures of water hardness range from soft to very hard, and the hardness of a water supply can have repercussions for the end user, depending on how the water is being used. Most commonly, people have issues with hard water, which leaves deposits on utensils, clogs pipes, causes soapy scums, requires more soap or detergent to clean objects, and may have a peculiar taste. Soft water, however, can also be damaging, most commonly by eating away at the tile around swimming pools.
For some applications, it is necessary that water be hardened by introducing a water hardener. This is most common in the case of swimming pools and aquariums. Aquarists frequently use the compound to meet the specific requirements of the fish they are raising. A water hardener is a calcium-heavy mixture that can be added to the water to increase its total hardness. It is important to be careful when using this compound, especially in swimming pools, because excessive use will make the water too hard, leading to calcium buildup.
A water hardener usually appears in the form of a powder, which can be measured out to increase the level of water hardness. At first, the compound may float on the surface or settle to the bottom in the form of grit, but as the water is circulated, the hardener dissolves into the water, raising the total level of water hardness. Depending on how many gallons of water are being adjusted, it may take up to a day for the water hardness to stabilize.
In the case of aquarists, two types of hardness are significant: general hardness (GH) and carbonate hardness (KH). Most consumers can use total hardness, which is a combination of the two. General hardness measures calcium and magnesium ions in the water, and although it is not the direct cause of pH imbalances, usually water with a higher GH has more alkalinity. An incorrect GH level may lead to difficulties with fish ranging from breeding difficulties to improper absorption of nutrients, and a hardener may be added to increase the GH.
Carbonate hardness refers specifically to the levels of carbonate and bicarbonate ions in the water. Carbonate hardness affects pH, binding with freed hydrogen atoms created by biological reactions. Normally, nitrogen levels in fish tanks increase as the fish excrete. This nitrogen breaks down into hydrogen and salts, which increase the pH in the aquarium over time. However, bicarbonate binds with hydrogen to keep the pH stable until the bicarbonate ions are used up, at which time a water hardener specialized for aquariums must be added to bring the pH down again.
In the case of fish keeping, care must be taken when adding a water hardener, because different GH and KH levels are required. In rare cases, a single water hardener can be added with safety. Most of the time, however, careful chemical adjustments are required to keep the water healthy for the fish living in it. This can be seen reflected at any commercial aquarium, where the levels of water hardness must constantly be adjusted and different species kept separate in water that maximizes their health.