Florida Department of Agriculture and Consumer Services
More confusion exists between hardness and alkalinity than any of the other water quality parameters. This is because most hardness and alkalinity comes from limestone or dolomite sources in nature. Limestone is calcium carbonate, and dolomite is a combination of calcium carbonate and magnesium carbonate. Water passes through the rocks in the ground as it makes it way to rivers and lakes, and picks up minerals on the way. When limestone and dolomite dissolve in water, one half of the molecule is calcium or magnesium (the "hardness") and the other half is the carbonate (the "alkalinity"), so most of the times they are equal. However, they are very separate measurements, and have very different importance for the aquarium.
Total Alkalinity: Total alkalinity is the measurement of all bases in the water and can be thought of as the buffering capacity of water, or its ability to resist change in pH. The most common and important base is carbonate. Total alkalinity is expressed as milligrams per liter (mg/L) or parts per million (ppm) of calcium carbonate (CaCO3). In the aquarium industry, total alkalinity may be referred to as "carbonate hardness" or "KH," which is often measured in degrees (dKH) rather mg/L or ppm. One dKH is equal to 17.9 mg/L or 17.9 ppm.
Waters that have moderate to high levels (50 mg/L or greater) of total alkalinity and total hardness (see below) usually have a neutral to slightly basic pH. The pH is more stable and does not change greatly throughout the day because the presence of carbonates and bicarbonates neutralize, or "buffer," the carbon dioxide and other acids in the water.
There are many reactions in aquariums that produce acids, and others, like biofiltration, that directly use carbonates. Over time the alkalinity can be "consumed," and if alkalinity is depleted, the pH of the water in an aquarium can plummet causing extreme stress or death to the fish and adversely affecting biofilter function. Alkalinity can be easily replenished in an aquarium by periodically exchanging a portion of the tank water with new water with a moderate total alkalinity or by adding chemical buffers, such as sodium bicarbonate (baking soda), to the water.
One place where total alkalinity is very important is when copper is used as a treatment for algae or parasites. Total alkalinity should be measured before copper is used in an aquarium, as the toxicity to fish is directly related to the total alkalinity. Copper should never be used with fish if the total alkalinity is less than 50 mg/L. Copper should not be used in aquariums with plants or invertebrates unless they are first removed from the tank. For more information on treating with copper, see the UF-IFAS publication "Use of Copper in Freshwater Aquaculture and Farm Ponds.".
Total alkalinity should be tested once a month to ensure levels are sufficient for the tank's fish inhabitants as well as for biofiltration. More frequent measurements (every week or every other week) may be necessary if alkalinity is less than 50 mg/L, stocking density or feeding rates are high, or if water changes are not done regularly. Prior to each copper usage, total alkalinity should be tested to prevent copper toxicity.
Total Hardness: Total hardness is the measurement of divalent cations (+2 ions) in the water and, like total alkalinity, is expressed as milligrams per liter (mg/L) or parts per million (ppm) of calcium carbonate (CaCO3). In the aquarium industry, total hardness may be referred to as "general hardness" or "GH," which is often measured degrees (dGH) rather than mg/L or ppm. One dGH is equal to 17.9 mg/L or 17.9 ppm. The two most common elements that contribute to hardness are calcium and magnesium.
Total hardness is particularly important when spawning fish and raising fry because calcium is critical to egg, bone and tissue development. However, some tropical aquarium species that originate in areas with extremely soft water may require low hardness water to spawn and develop, so it is important to know the specific requirements for each species that will be spawned or maintained. Hardness ranges from 10mg/l to over 400mg/l depending on the region it comes from. Tropical rainforest rivers like the Amazon may have less than 10 mg/l, and the rift valley lakes of Africa can reach 500mg/l.
Total hardness should be tested once a month in most aquariums. However, if one is breeding fish or raising fry, total hardness should be tested more frequently.