Glossary Of Terms Used In Fish Keeping

Glossary Of Terms

When you first enter the field of aquatics you soon discover that there are words and terms being used that you do not fully understand.

To help you we have created a Glossary section, with a list of words commonly used, along with a brief description explaining their meaning.

Hopefully this will not only give you the confidence to be able to ask the right questions, but that you will also be able to understand the answers!


Water with a pH reading of less than 7


A clear plastic material that is used to make aquariums. It has similar properties to glass, but is much lighter.


A fluorescent bulb that emits blue colored light, which is often used in marine tanks

Activated Carbon

Used in a filter to to remove waste including organic or medications

Adipose Fin

A small fin possessed by some fish. It is located behind the dorsal fin


Existing or active in the presence of free oxygen.

Air Stone

A block with minute holes that is used with an air pump to diffuse air

Algal Bloom

A rapid growth of algae


Water with a pH reading of greater than 7


The toxic byproduct produced by fish metabolism or decaying organic materials


A fish born in fresh water that then migrates to a saltier environment Catadromous.


Existing or active in the absence of free oxygen.

Anal Fin

The fin that is found behind the anal opening


Low oxygen

Aquarium Salt

A salt that is used in freshwater tanks to treat disease. This is not the the same as marine salt!


Arranging plants and decorations in a tanks, the equivalent of landscaping


A biological filter


Abbreviation of biological load, which is the fish waste that needs to be processed by the filter

Blood Worms

Larvae from the family Chironomidae, used as fish food


Wood that has been preserved in a peat bog.


Water that is slightly salty


A fish born in salt water that then migrates to fresh water

Caudal Fin

The tail fin


A group that includes amongst others Tetras and Piranhas.


A chemical element that is frequently added to domestic water supplies. This can prove toxic to fish. Chlorine dissipates from water if it is left to stand for a day, or it can be removed by using a dechlorinator


A deposit of fish eggs

Community Tank

An aquarium populated with fish from several different species


Establishing the nitrogen cycle in an aquarium

DI Water

Water purified by deionization


A freshwater crustacean used as as fish food


Active during the day

Dorsal Fin

A fin on the top of many fish.


Fishes body becoming swollen due to fluid accumulation. This is a symptom of various diseases


The device used to remove waste particles and impurities from water

Filter Media

Materials which are used in a filter to remove particles and impurities

Fishless Cycle

Establishing the nitrogen cycle in an aquarium without the use of fish


Newly hatched fish


A female fish who is ready to spawn.

Hard Water

Water containing a high proportion of dissolved minerals and solids.


An instrument used to measure the salt content in water


Abbreviation for invertebrates

Labyrinth organ

An organ that allows fish to obtain oxygen from the air


A saltwater aquarium

Median Fins

Fins that are not paired


Fish semen


Fish which protect their eggs or fry /or young by holding them in the mouth

Nano Reef

A small reef aquarium.

Nitrogen Cycle

The process by which bacteria convert fish waste and other decaying organic matter into less harmful compounds

New Tank Syndrome

A tank where the nitrogen cycle has not yet been properly established


Active at night

Old Tank Syndrome

A tank suffering from a decline in pH due to the the accumulation of excess nitrates


An animal that eats both meat and vegetable based foods


The organ through which a female fish deposits her eggs


A measurement of the degree of acidity of water. pH 7 equals neutral water. Below this it is acidic, and above it is alkaline


An aquarium that also has a land area


A tube on a male fish used to fertilize eggs

Pectoral Fin

Fins based on the sides of the fish.

Power Head

A submersible pump

Protein Skimmer

This is used in saltwater tanks to remove organic pollutants

RO Water

Water that has been purified by reverse osmosis

Reef Aquarium

A marine aquarium containing live corals and normally reef fish and invertebrates


The amount of salt in the water

Soft Water

Containing a low proportion of minerals and solids

Species Tank

An aquarium that houses only one species as opposed to a cmmunity tank


Material at the bottom of the aquarium eg: sand or gravel

Swim Bladder

The internal air sac which can contains controls a fished buoyancy

Trickle Filter

A type of bio-filter in which water is trickled over the filter media


The rate at which the total volume of the tank water is processed through the filter system

Ventral Fins/Pelvic Fins

The last set of paired fins on the underneath of a fish

Water Column

The calculated from water surface to the top of the substate

Water Conditioner

A chemical used to make tap water safe for aquarium use

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