What Is the Desired Calcium Level for an Aquarium?

Fish need calcium for strong bones, and crustaceans and mollusks need calcium to build shells and carapaces. Calcium plays an important part in the overall water chemistry of an aquarium or natural environment. Depending upon the type of aquarium and the species of fish you have, you can measure calcium in the form of calcium ions or water hardness and adjust the levels to meet the needs of your fish.

  1. Calcium Levels

    • Freshwater fish tolerate a wide range of calcium, and the optimum level depends on the type of fish in the aquarium. Most people who keep freshwater fish measure the general hardness, which is a combined measurement of calcium and magnesium, rather than the actual calcium because it is more relevant to the needs of the fish and stability of the aquarium. In saltwater and brackish water aquariums, the calcium level should closely match that found in nature. That means a suitable calcium level is around 420mg per liter in a saltwater or reef aquarium and 200mg per liter in brackish water.

    Measuring Calcium

    • Any good pet or aquarium supply store will have calcium test kits. Carefully read and follow the instructions included with your kit, and follow them precisely. Kit manufacturers sometimes change the kit methodology, so reread the instructions each time you purchase a new kit. Clean your testing equipment by rinsing it thoroughly in aquarium water and drying it with a paper towel to remove residual reagents. Never use soap. Store the kit in a cool, dry place to prolong its shelf life.

    Calcium and Hardness (GH)

    • General hardness (GH) measures the calcium and magnesium ions in the water. Don't confuse this test with carbonate hardness (dH or dKH), which measures hardness in a different way, and indicates the buffering capacity and pH stability of the water. The measurement of the actual calcium isn't as important in freshwater aquariums as it is in other systems, so measuring the general hardness is enough to keep your fish happy and healthy. General hardness is measured in parts per million (ppm). A ppm of 0 to 70 is very soft water, and a ppm of 321 to 530 is very hard water. The desired ppm depends on that required for the health of the inhabitants of your tank.

    African Cichlids

    • African cichlids have different water requirements from other freshwater fish.

      African cichlids come from three lakes in the African rift valley: Lake Victoria, Lake Tanganyika and Lake Malawi. The water chemistry in these three lakes is different than that of any other natural body of water, and the aquarium hobbyist must try to duplicate the chemistry as closely as possible. African cichlids need a very high pH, and one way to stabilize the pH is to use coral sand, crushed shells or dolomite as a substrate instead of gravel. These substrates are high in calcium -- but in the form of calcium carbonate, which converts to useable calcium very slowly. For African cichlids, you are better off monitoring carbonate hardness rather than general hardness or calcium ions. The carbonate hardness should be between 5 dKH and 8 dKH for Lake Malawi cichlids, between 16 dKH and 19 dKH for Lake Tanganyika, and between 2 dKH and 8 dKH for Lake Victoria species.