Kalkwasser: A German term that means literally chalk-water, this is water with calcium hydroxide Ca(OH)2 dissolved in it, and has a pH of approximately 12.0. Adding kalkwasser is the preferred way to maintain calcium levels in a marine tank with growing corals, clams, and calcareous algae. About 1 teaspoon of Ca(OH)2 powder will dissolve in a gallon of pure water.

Kelp: Any of several large, brown algae, including the largest known algae.

Kelvin: A temperature reading used to rate the color of light bulbs. 5500 degrees K is equal to sunlight.

Keystone Species: A predator at the top of a food web, or discrete sub web, capable of consuming organisms of more than one trophic level beneath it.

Killifish: This family of small freshwater fish is quite rare in the fish hobby. They live only one year, laying eggs that can then survive harsh conditions.

King: The name given to the chinook salmon because it is the largest species of salmon in the world. King salmon are found in the Chena.

Krill: A term used by whalers for the small, shrimplike crustaceans found in large masses in polar waters and eaten by baleen whales. This is a popular food to feed both freshwater and marine fish.