Kalkwasser: Calcium hydroxide in water. Used in marine tanks to replace calcium used by hard corals and clams to build calcerous skeletons. The preferred way to maintain calcium levels in a marine tank.

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

Labyrinth fish: See Anabantoids.

Labyrinth Organ: An organ located above the eyes in anabantoids, which enable them to take in oxygen from the air.

Lateral line: A line of sensory scales along the sides of fish that enables them to detect vibrations in the surrounding water caused by other fish and their own reflected vibrations against obstacles.

Laterite: Substrate additive used in planted tanks to supply nutrients.

LFS: local fish store/shop

Littoral: Living near edge of a lake, near the shore.

Live rock: Rocks removed from a tropical coral reef with all of the life intact on them - including algae, sponges, worms, feather dusters, small crustaceans, polyps and urchins. An important part of most marine tanks and all reef tanks.

Livebearer: A fish whose eggs are fertilized and hatched inside the body of the female. Includes guppies, mollies, platys, and swordtails.

LLE: Lateral Line Erosion.

LPS: Local pet store/shop.

LR: Live rock.

LS: Live sand.

Lumen: A measurement of light brightness or intensity.

Lux: The standard for measuring light.

Macro-Algae: Complex celled algae found in marine aquaria.

Mbunas: Common name for rock dwelling African Cichlids from Lake Malawi. Literal translation: "rock fish".

Mechanical filtration: Filtration that mechanically removes particles by passing water through a porous substance such as a sponge, floss or mesh.

Metal Halide: A very intense type of lighting used in marine and freshwater plant aquaria. The primary drawback (aside from expense) is that they are very hot.

MH: Metal halide (lighting)

Micro-Siemens/cm: A unit of electrical conductance: how easily electricity flows through a material (in this case, water).

Milt: The sperm of a male fish.

Mouth brooder: Fish which protect unhatched eggs and young fry in the mouth.

MTS: Malaysian Trumpet Snails (or Multiple Tank Syndrome, a malady common among fishkeepers for which there is no known cure).

Nanometer (nm): A unit of measurement of the wavelengths in the light spectrum.

Nematocysts: The cells at the tip of anemones' (corals) tentacles, which "sting" when touched.

Nitrate (NO3): The final stage of nitrification. Mildly toxic , nitrates are removed with regular water changes or by plants. Ideally kept below 40 ppm.

Nitrification: The process by which ammonia is changed to nitrite, then nitrate. Also called the nitrogen cycle. Ammonia created by urea and decomposition is turned into nitrites by nitrosomas bacteria. Nitrites are converted to nitrates by nitrobacter.

Nitrite (NO2): N02 is converted from free ammonia during nitrification and is harmful at any level to most creatures. Should be below detectable levels in a cycled tank.

Nitrogen Cycle: See nitrification .

NO: Normal output (lighting).

NTD: Neon Tetra Disease. A parasitic disease with a high mortality rate and no known cure. Although first noticed in Neons, many fish are susceptible to one degree or another.

Nuchal Hump: An enlarged forehead on some male cichlids.

O.D.: "Outside diameter", used when measuring tubing dimensions.

Omnivore: An organism that eats both meat and vegetable matter. Most freshwater fish fall into this category.

Oodinium: "Velvet disease" A freshwater parasite which gives the fish the look of being dusted with gold dust. Copper is the usual treatment.

Operculum: A hard, bony plate, which covers the gills.

Oviparous: Eggs, which are fertilized, develop and hatch outside the body.

Ovoviviparous: Producing eggs that are fertilized internally. Hatching may occur internally or externally

Ozone (O3): Very reactive gas used as a sterilizing agent. Care must be used in its use, as excess ozone can harm fish and other creatures.