Glossary of Aquarium Terms

This glossary of aquarium terms contains definitions for some of the most common words used in the hobby.


Acclimate - The process of allowing an inhabitant to move from one water source to another (such as moving from the pet store's tank to your aquarium).

Acidic - Relates to the pH scale in which anything that is measured below a pH of 7.0 is considered an acid (or acidic).

Aeration - Refers to increasing the amount of air that is passed through the water in an aquarium (also known as creating surface agitation to do so).

Aerobic - Requires the presence of oxygen to survive.

Ahermatypic - Relates to corals that do not have zooxanthellae and are generally not reef-building coral.

Algae - A simple non-flowering aquatic plant of a large group that contains chlorophyll but lacks true stems, roots, leaves, and vascular tissue. Some algae can be beneficial while others can be labeled as a nuisance/issue.

Alkaline - Relates to the pH scale in which anything equal to or above a pH of 7.0 is considered a base (or basic/alkaline).

Alkalinity - The capacity of something to resist a downward change in pH (normally referred to as carbonate hardness).

Ammonia - NH3 is a toxic chemical byproduct produced during the breakdown of proteins in uneaten fish food, decaying waste, or excreted by aquatic animals inside of the aquarium.

Anaerobic - Refers to something that can survive in conditions without any oxygen.

Aragonite - A calcium-based material that is used in substrates of aquariums.

Asexual - An organism that has no sexual organs, and therefore is neither male nor female.

Autotrophic - An organism that is capable of synthesizing (creating) its food from inorganic substances.


Bacteria - Refers to microorganisms that are found, some can be beneficial (ex; Nitrosamines and Nitrobacteria in breaking down waste) or harmful (those that act as pathogens attacking fish with infections or diseases).

Benthic - The bottom of a body of water, such as the seafloor or the bottom of a lake where flora and fauna are found.

Bioball(s) - A biological media that is used in filters. Normally are made of plastic, in a spherical shape, and have designs on the outside to attach the growth of beneficial bacteria.

Biological Filtration - The use of filter media to grow colonies of aerobic and anaerobic bacteria to remove ammonia and nitrite from the water.

Bioluminescent - Microbes and animals that can produce their light.

Biomass - Refers to the total mass of living matter within a specified area (normally referred to as bioload).

Biomechanical - The use of filter media that contains mechanical and biological filtration inside one unit.

Bleaching - The process in which zooxanthellae expel themselves from corals causing a lack of pigmentation in the given coral.

Blushing - A pink or reddish appearance normally associated with a fish.

BPM - Abbreviation for Bubbles Per Minute, a term used to measure the amount of carbon dioxide bubbles that are released into the water when using a CO2 system.

Brackish - Water that has a slight salt content with a specific gravity between 1.006-1.015. Found naturally where rivers meet the ocean.

Bubble Nest - Appears like small bubbles that look like foam, commonly created when there is a large protein content in the water or created by anabantoid fish when they are searching for a mate.

Buffering - The capability of something to resist changes in pH.


Carnivore - An organism that primarily feeds on a meat-based substance for its diet.

Chemical Filtration - The process of removing particles in the water through chemical reactions (commonly done by using carbon or resins).

Chiller - Equipment that is used to cool down water (opposite of a heater) when it reaches a certain temperature, frequently used with saltwater-related setups but also can be found in freshwater setups dependent on the environment and species that are housed within.

Cycling - The term used for the process of creating The Nitrogen Cycle within an aquarium, which creates nitrifying bacteria.


Dechlorinator - A liquid chemical that is used to remove any chlorine and chloramine that is found within tap water, to make it safe for aquatic life.

Deionization - A process where nitrates are converted into nitrogen gas, that escapes through the normal gas exchange process at the water surface.

Detritus - Decomposing organic matter, such as fish waste, plant leaves, or excess food that is uneaten.

Dither Fish - These are fish that swim around the tank and in the open, and in the process, will calm and relax fish that may be timid in nature.

Diurnal - A plant, or animal that is active during the daytime when the sunlight is out.

DIY - Abbreviation for Do It Yourself, common when saying something is able to be completed by anyone regardless of skill or prior experience.

dKH - Abbreviation for Degrees of Carbonate Hardness and is the measurement of total alkalinity.

DOC - Abbreviation for Dissolved Organic Carbon or Dissolved Organic Compounds. Dissolved carbon compounds form naturally within the aquarium as a biological process that takes place as compounds dissolve into simpler compounds. Commonly is tested by measuring Nitrate levels, or can be tested on its own.

Dorsal Fin - The fin that is located on the top of fish, although some species may have multiple dorsal fins.

Doser - A liquid distribution device that is either gravity or mechanically managed to maintain specific water parameters or levels.

DSB - Abbreviation for Deep Sand Bed.


Endemic - The species is native to that specific place or geographic location.

Endosymbiosis - When an organism lives within the cells of another organism.

Euthanize - To terminate a species in a painless manner in order to stop it from suffering during death.


Ferts - Abbreviation for Fertilizers.

Filter - Equipment that is used to remove various levels of impurities from aquarium water through biological, chemical, and/or mechanical methods.

Fry - Recently hatched fish are commonly called fry due to their small appearance.

FO - Abbreviation for Fish Only.

FOWLR - Abbreviation for Fish Only With Live Rock.

Flashing - A term used to describe the quick darting nature when a fish rubs or scrapes itself against objects within the aquarium.

Fluidized Bed Filter - An aquarium filtration device that forces water through a sand-based medium.

Frag - Abbreviation for a coral fragment, which refers to a small piece of coral that has been broken off.

Fry - A term used to describe baby fish, that have not yet hit maturity.

FW - Abbreviation for Fresh Water.


GAC - Abbreviation for Granular Activated Carbon.

Gang Valve - A valve that is used within an air pump line to direct and/or regulate airflow.

GH - Abbreviation for General Hardness, a measurement of total calcium and magnesium.

GFCI - Abbreviation for Ground Fault Circuit Interrupter, a safety device on an outlet that takes sure if there is a difference in electrical flow that it will trip and prevent an under or overload on the devices connected to the outlet.

GPD - Abbreviation for Gallons Per Dat.

GPH - Abbreviation for Gallons Per Hour.

Gonopodium - A modified anal fin that male fish have, used for reproductive purposes.

Gravel - A type of aquarium substrate that is on the bottom of the tank.


Hard Water - A term used for water that contains various many minerals, such as calcium and magnesium.

Herbivore - An organism that primarily feeds on a vegetable-based substance for its diet.

HID - Abbreviation for High Intensity Discharge, which refers to lighting fixtures that produce high-intensity lighting.

HO - Abbreviation for High Output, commonly refers to lighting fixtures that produce high-intensity lighting.

HOB - Hang On Back, commonly associated with filters that physically hang on the back of the tank trim.

Hydrometer - A highly sensitive device that measures the specific gravity and salinity of the water. Commonly used in saltwater-related setups.


Ichthyologist - The study of fish species.

Internal Filter - A filtering device that is placed within the aquarium, in either a partially or fully submerged state.



KH - Also referred to as dKH, is a measurement of bicarbonates and carbonates and is also known as total alkalinity or acid-neutralizing capacity.


Lateral Line - Fish can use their lateral line in order to detect movement within the water, used to detect fish, food, and other objects.

LFS - Abbreviation for Local Fish Store (or Shop).

LPH - Abbreviation for Litres Per Hour.

Light Meter - A highly sensitive piece of equipment that measures the intensity of light underwater.

Livebearer - Fish species that have the ability to give birth to free swimming fry, compared to eggs.

Live Rock - Rocks that have broken off of a coral reef structure, and is inhabited by many forms of life and organisms that are highly beneficial to saltwater tanks.

Lumens - The measurement used for the total output of any given light source.


Magnesium - Element Mg, is the third most abundant element in the natural ocean and is used by algae and other microorganisms.

Mantle - Refers to the top, fleshy part of a clam species.

Mbuna - A common name for African Cichlids that are from Lake Malawi.

Mechanical Filtration - The process of removing particles in the water through mechanical processes by using various filter media to remove debris, fish waste, uneaten food, and other particle matters.

MTS - Abbreviation for either Multiple Tank Syndrome, or Malaysian Trumpet Snail.

Mutualism - A form of symbiosis in which both organisms derive some mutual benefit from each other.


Nanometer - Refers to the wavelength of light within an aquarium lighting fixture.

Nitrate - A toxic byproduct of the nitrogen cycle that is the least toxic nitrogen byproduct often used by live plants.

Nitrite - A toxic byproduct of the nitrogen cycle that is often broken down and used by plants in a planted aquarium or can be removed with water changes.

NO2 - Chemical notation for Nitrite.

NO3 - Chemical notation for Nitrate.

Nocturnal - A plant, or animal that is active during the nighttime when the sunlight is gone.


Omnivore - An organism that feeds on both meat & vegetable-based substance for its diet.

Ozone - A very unstable gas that is used to increase the dissolved oxygen content within an aquarium.


PAR - Abbreviation for Photosynthetically Active Radiation, the wavelength of the light spectrum that is created from your aquarium light (or the sun) and is normally measured from under the water to the bottom or wherever you are planning to keep any plant/coral species.

pH - Measurement of relative alkalinity or acidity within a specific type of liquid. A pH reading of 7.0 is considered neutral, higher than 7.0 being a base/alkaline, and readings lower than 7.0 are acidic.

PPM - Abbreviation for Parts Per Million, which is used as a measurement of a particular element commonly used in test kits.


Quarantine - The process of dividing newly acquired species or ill species by isolating them in their aquarium for treatment until they are fully cured.

QT - Abbreviation for Quarantine Tank.


RCS - Abbreviation for Red Cherry Shrimp.

Reef Tank - A saltwater aquarium that is used to keep saltwater corals, and other invertebrates only.

RO - Abbreviation for Reserve Osmosis, which is water that has been fully stripped of all minerals and elements, to only contain pure H2O. It is commonly used to replace evaporated water.

Rock Scape - The process of decorating and/or arranging Live Rock within an aquarium.


Siphon - Refers to a hose that is commonly used during water changes that use either gravity or suction pressure to transfer water from your aquarium into a bucket or other location.

Specific Gravity - The measurement of aquarium water's relative density or salt content.

Substrate - The material that is placed on the bottom of an aquarium (gravel, sand, pebbles, etc.) that can provide a functional, chemical, and cosmetic purpose.


Tannins - Comes from leaves, driftwood, or other sources that change the water color to a yellowish tint and will naturally lower pH.

TDS - Abbreviation for Total Dissolved Solids.

Turn Over - Refers to the amount of water that is processed through a filter, or movement of water through a device normally in the terms of GPH.


Undergravel Filter - A filter that is placed underneath a layer of substrate (cannot be sand) that is powered by an air pump or powerhead to suction water into the gravel to use as a biological and mechanical filtration system.


VHO - Abbreviation for Very High Output.


Water Change - The process of removing old aquarium water, and replacing it with a new source of water.

Water Parameters - Refers to specific chemical parameters for maintaining a healthy aquarium, and keeping the aquarium cycle.

Wet/Dry Filters - A filter that exposes beneficial bacteria to both water (wet) and air (dry) for increased biological filtration.




Zeolite - A mineral that is used in the chemical filtration of some aquarium filters.