Title graphic for Sea and Sky's Saltwater Aquarium Guide

Glossary of Aquarium Terms

Keeping a marine aquarium can be a rewarding and fascinating hobby. The saltwater aquarium hobby contains words, labels, and technical terms that may seem be a little confusing for both beginners and even the more experienced hobbyists.

This glossary of aquarium terms contains definitions for some of the most common words used in the home aquarium hobby. You can click on any letter of the alphabet below to jump directly to that section in the glossary listing.



Actinic Lights
A type of florescent light with a very blue spectrum. It is the primary color of light in the ocean below 30 feet and is required by corals and other reef creatures which contain photosynthetic algae.

Activated Carbon
A form of carbon specially formulated for filtration. Carbon is good for removing a large number of toxins and other unwanted substances from aquarium water. it is especially useful for clearing or "polishing" cloudy water. One problem with carbon is that is can release phosphate into the water, which stimulated algae growth.

Requiring the presence of oxygen.

Aerobic Bacteria
Bacteria that requires oxygen to survive.

Air Pump
A pump used to aerate water by pumping bubbles through it. Air pumps are usually used in conjunction with an airstone to create bubbles in the water.

is a device fitted onto the end of a length of air-hose that typically diffuses air into the water. A typical airstone is a porous blue or green ceramic stone with a plastic nozzle that is used to connect to an air pump.

Underwater growths of plant-like organisms. Some algae may resemble plants but they are actually not plants at all. Algae ranges in type from tall stalks of kelp to fuzzy growths of green hair algae to encrusting growths of coralline algae.

a chemical that is used to control or kill algae.

A term used by aquarists to refer to the ph value of water. Water with a ph higher than 7 is said to be an alkaline.

The capacity to buffer against pH drops. The greater the alkalinity, the more stable the pH will be and the less likely that there will pH swings. Alkalinity can be raised by adding a carbonate buffer material. Alkalinity can also be maintained through the use of substance called kalkwasser.

NH3, a toxic substance that builds up in the aquarium. It is released by fish through their gills and as a result of waste buildup. Ammonia is the first step in the nitrogen cycle, and is removed by bacterial action where it is transformed into nitrite, or can be removed by mechanical filtration.

A lack of oxygen. Anaerobic zones in an aquarium are areas where no oxygen is present such as inside live rock or under sand or gravel. Anaerobic bacteria live in these areas where they transform nitrate into nitrogen gas. These areas can also produce hydrogen sulfide and other toxic substances.

Anaerobic Bacteria
Bacteria that do not require oxygen to survive. Oxygen is actually toxic to some types of anaerobic bacteria. Anaerobic bacteria in the aquarium transform nitrate into nitrogen gas, effectively removing nitrate from the water.

The craft of arranging aquatic plants, as well as rocks, stones, cavework, or driftwood, in an aesthetically pleasing manner within an aquarium. The goal of aquascaping is to create a realistic, natural-looking habitat for aquarium animals.

A calcium-containing mineral usually found in the form of rock, gravel, or sand. Aragonite is a popular substrate for marine and saltwater aquariums.


Bacteria are microscopic, single-celled organisms. In the aquarium hobby, beneficial bacteria can be used to filter the water by breaking down and consuming waste products.

A power source required for fluorescent and metal halide lights. They are highly specialized and each type of light requires its own type of ballast.

Berlin System
A method of biological filtration for a marine aquarium that uses only live rock and a powerful protein skimmer.

Bio Balls
Plastic, hollow balls of various sizes which are used for biological filtration in an aquarium. They usually have complex shaped that provide a large surface area for beneficial bacteria to grow.

A term used to describe the amount of life existing in an aquarium. The bioload of an aquarium is dependent upon the ability if the filtration system to remove waste materials from the water.

Biological Filtration
A method of natural filtration for an aquarium that uses bacteria to break down waste substances by means of the nitrogen cycle. These include undergravel filters, trickle filters, and sponge filters.

A mollusk or other shelled animal who's shell is comprised of two separate halves, or valves, usually connected by a flexible hinge.

A process by which corals expel their colorful zooxanthellae and turn white or pale. Bleaching is commonly caused by pollution and warming of the ocean water.

Blue Green Algae
A form of cyanobacteria that forms a blue-green slime that covers surfaces in an aquarium.

Brackish Water
Water that is half fresh and half salt. There are number of species, such as brine shrimp, that prefer this sort of diluted salt water.

Brine Shrimp
A tiny species of shrimp growing to only about 1/4 inch. Also known as "Sea Monkeys", they are sold as a source of fish food. Brine shrimp make a delicious snack for reef fishes, but are not very nutritious and should not be used as the sole food source.

A substance added to the aquarium water to raise the alkalinity or adjust the pH. Several different types of buffering materials are available. Some can be used to raise or lower pH, and some can raise alkalinity without affecting pH.

Byssus Gland
A specialized gland found in bivalve mollusks, such as clams, that produces sticky threads used to secure the animal to rocks.


Any substance formed of or containing calcium carbonate. Calcium carbonate can help maintain a high pH in aquarium water.

A process by which corals and coralline algae extract calcium from the seawater and deposit it in the form of calcium carbonate.

A mineral that is the major building block of corals and other calcareous organisms. In a reef tank, calcium levels should be maintained between 380 and 480 mg/l. Calcium levels can be maintained through regular water changes, by using calcium additives, or through the use of kalkwasser.

Calcium Carbonate
(CaCO3) a chemical compound commonly used as a calcium supplement. used in aquariums as a principle method to increase the general hardness (GH) of the water.

Calcium Hydroxide
Ca(OH)2 a substance mixed with water and dripped into the aquarium to maintain calcium, pH, and alkalinity levels. See kalkwasser.

Cannister Filter
A filtration system that consists of an external cannister that contains various mechanical filtration media. Water is pumped out of the tank, forced through the cannister, and then returned to the tank.

A substance used for filtration in an aquarium. See activated carbon.

Check Valve
A simple device used in air tubing to prevent water from running backwards down an air tube. A check valve is used with an air pump to prevent the water from flowing back down the tube and potentially flooding the room.

Chemical Filtration
A method of filtration that uses chemical processes to clean the water. Examples of this type include activated carbon and protein skimmers.

A piece of equipment used to cool down the water in an aquarium. Chillers are available in different types and sizes, including one that hooks up in-line with the water flow of the tank and one that drops into the sump. They all feature a thermostat for maintaining a constant temperature. Larger tanks require larger chiller units.

Chitin is the semi-transparent material that makes up the exoskeleton of shrimps, crabs and other invertebrates.

A chemical commonly used in low concentrations as a disinfectant in municipal water systems. Chloramine is toxic to marine life and must be removed from the water by filtration before the water can be used in an aquarium.

A substance used in municipal water supplies to kill bacteria. Chlorine is toxic to fish and invertebrates and must be removed from water before it can be added to the tank. A number of products are available for this purpose.

A green photosynthetic pigment found in most plants, algae, and cyanobacteria. Chlorophyll allows the organism to use light to produce food through a process known as photosynthesis.

Color pigmentation cells in the skin of animals. Chromatophores are used by a number of fish and invertebrates change their colors to blend in with their surroundings and hide from predators.

A phylum of animals that contains jellyfish, sea anemones and coral polyps.

Tiny crustaceans found in a variety of aquatic habitats. Copepods are able to reproduce in an aquarium and make an excellent source of live food for aquarium fish.

A metal used in the form of copper sulphate to cure diseases and parasite infestations in the aquarium. Copper is highly toxic to marine invertebrates and should NEVER be used in a reef tank.

Any one of a group of marine invertebrates in the class Anthozoa of phylum Cnidaria. Corals typically live in compact colonies of many identical individual polyps. Some corals have soft bodies while others secrete calcium carbonate to form a hard skeleton.

Coral Reef
A diverse underwater ecosystem anchored by large calcium carbonate structures secreted by hard corals. Coral reefs provide a home for at least 25% of all marine species including colorful fish and invertebrates.

Coralline Algae
An encrusting form of algae that forms calcareous crusts like coral. Coralline algae is very colorful, occurring in bright purple, pink and red colors. It is very desirable in the reef tank, and can be made to grow on rocks and other hard surfaces by maintaining optimum, alkalinity, and calcium levels.

Coral Sand
A white or light-colored medium composed of fine crushed coral and shells and sometimes aragonite. The shells are made primarily of limestone and calcite.

A group of hard-shelled invertebrates that includes crabs and shrimps.

Microscopic, photosynthetic organisms that can form large colored mats. They are usually blue-green in color, but can also be found in dark green, red, brown, or black.


An organism that commonly forms brown films on aquarium glass or rocks. Diatoms form their shells from silicate, and can be controlled to some degree by preventing the addition of this compound through the use of purified water.

A chemical or treatment that used to remove toxic chlorine from water. A dechlorinator should always be used before adding tap water to an aquarium.

Deionized Water
Water that has been pushed through resin which removes most of the dissolved minerals and metals from the water. The result is nearly pure water with a pH close to 7.

A device for filtering water that uses several ion exchange resins to purify and remove impurities from the water.

The process by which nitrate is converted into nitrogen gas and released into the atmosphere. In the aquarium, denitrification is performed by anaerobic bacteria.

Waste material that accumulated in gray piles in the aquarium. Detritus is high in nutrients and should be removed when possible to help prevent the growth of unwanted algae.

Dissolved Oxygen
The amount of oxygen currently dissolved in a body of water. Marine animals require dissolved oxygen in order to survive. Dissolved oxygen concentrations can be determined with test kit or by measuring electronically with an oxygen meter.

Distilled Water
A type of pure water, also known as deionized or RO water, that has been cleaned and purified to remove at least 99.5% of all impurities. Distilled water is preferred for maintaining a healthy aquarium environment.

Dorsal Fin
The fin located on top of a fish. Most fish species have only one dorsal fin, but some will have two, one behind the other. Many species of clownfish will have two dorsal fins.

Dosing Pump
A device used to deliver small amounts of chemicals and trace elements into the aquarium water. It is recommended that kalkwasser be dosed in this manner.

Dyed Fish
A fish that has been artificially colored or enhanced by injection of dye or paint or by use of lasers. Dying is a cruel practice that stresses and kills many fish and shortens their life. Dyed fish should be avoided and the practice discouraged.


A parasite that lives on the surface of its host. In aquarium fish, ectoparasites can typically be found on the skin, fins or gills.

External Filter
Any filter not kept inside the aquarium, but connected with hoses or tubes. Canister filters are an example of an external filter.


Any device used to remove unwanted particles or compounds from aquarium water. Filters come in a variety of styles, but most fall into three main categories: biological, chemical, or mechanical.

Filter Feeder
An organism that feeds by filtering out nutrients such as plankton, bacteria, or detritus from the seawater.

Filter Medium
Any substance used in water filtration systems to remove organic wastes and impurities from the water.

A common bacterial disease of fish kept in low quality water conditions. As the disease progresses, the area of the damaged fin becomes opaque and white then is eaten away by the bacteria.

Any member of a group of water-dwelling, mostly cold-blooded vertebrates with gills. There are over 29,000 species of fish, which makes them the most diverse group of vertebrates in the world.

Flake Food
A type of food for aquarium fish that has been processed into thin layers or flakes which float on the surface of the water.

A variety of small, flattened, aquatic worms. In aquariums, flatworms are considered pests because they can destroy corals and other delicate invertebrates.

Fluidizing Bed
A method of biological filtration where water is forced through a cylinder containing small beads. Nitrifying bacteria growing on the beads removes waste materials from the water.

Foam Factionation
A method of removing proteins from water through the use of foam. This is the filtration method used by protein skimmers.


General Hardness (GH)
A measure of the overall concentration of calcium, magnesium, and other ions. It is measured in degrees, with one degree equal to about 17.9mg/l (17.86mg/l). The harder the water, the higher the GH number.

The membranes through which fish absorb dissolved oxygen from the water during respiration.


Halogen Lights
Lights with a very yellow color spectrum. Due to their color, these lights are not recommended for use in a reef aquarium.

See General Hardness.

A device used to heat the water in an aquarium. Heaters vary in size and style including drop-in types and submersible sump types. They feature an adjustable thermostat to maintain the water at a constant temperature. The size and wattage of a heater required will depend on the water volume of a tank.

Heavy Metals
Chemical elements such as cadmium, copper, lead and zinc. Heavy metals dissolve readily in water and are toxic to marine life. Copper is one of the most common heavy metal found in water due to copper pipes in older homes.

An animal that eats plants. Herbivores such as snails and tangs are an important part of a reef tank because they help keep algae under control.

Hospital Tank
An aquarium set up separately from a main or display system used to house new livestock. Also known as a quarantine tank, it is used to prevent spreading diseases into an established aquarium.

A device used to measure the specific gravity of seawater. The most common types consist of a clear chamber with a floating needle.

Hypoxia occurs when an animal is unable to get enough oxygen. Hypoxia in the aquarium can be deadly and can be caused by faulty filters, an over abundance of algae, or too many animals in the aquarium.


An electrically operated propeller that causes water to flow through a pump or filter.

An animal with no backbone or skeleton. This group of animals includes mollusks, crustaceans, worms, and corals. A large percentage of animals on coral reefs are invertebrates.

A trace element found in seawater necessary in small quantities for some reef invertebrates, particularly corals and clams.


Jaubert System
A method of biological filtration that uses only live sand and a powerful protein skimmer.


A saturated solution of calcium hydroxide (Ca(OH)2) and water. Kalkwasser can be dripped into the aquarium or mixed with top off water to help maintain calcium, pH, and alkalinity levels. It is an extremely potent substance and should be used with caution. An overdose can raise alkalinity levels as high as 14 and can be harmful to animals.

Tiny, shrimp-like marine invertebrates. They are important part of the marine environment, providing food for baleen whales, manta rays, and other species. Krill is often sold in aquarium stores either as live food, frozen, or freeze-dried.


The first stage of development after hatching for many fish and invertebrates.

Lateral Line
A line of perforated scales along the side of a fish that is connected to a specialized organ used to sense vibrations in the water.

An acronym used by aquarium hobbyists that stands for Local Fish Store.

Live Food
A type of food for aquarium fish that is sold while still alive. Some predatory fish such as lionfish will only eat live food. Live food for the aquarium can include brine shrimp, copepods, small shrimp, and other small invertebrates.

Live Rock
Rocks removed from the ocean that usually have a variety of sea life attached to them, including nitrifying bacteria, sponges, algae, coralline algae, worms, and starfish. Live rock is commonly used in reef aquariums because it contains bacteria that can help filter the water through nitrification.

Live Sand
Sand removed from the ocean that contains nitrifying bacteria and other sea organisms. Live sand is used in reef aquariums because it contains bacteria that can help filter the water through nitrification.


Large plant-like algae commonly found in red, green and brown varieties. One of the most common of these is Caulerpa, which produces large green spheres resembling grapes.

Marine Aquarium
An aquarium that keeps marine plants and animals in a contained environment. Also known as a saltwater aquarium, a marine aquarium is usually set up to reproduce a marine environment such as a coral reef.

An elemental metal that plays a critical role in the chemical and biological processes in the marine aquarium. Magnesium is important for the the skeleton forming process of stony corals and other invertebrates.

Mechanical Filtration
A water filtration method that uses filtering medium to remove particles from the water. Cannister filters, undergravel filters, and wet/dry prefilters are examples of mechanical filters.

See Filter Medium.

Small microscopic types of algae such as the green algae and hair algae common in marine aquariums.

Metal Halide
A type of light bulb that uses special gases to give off a very bright white light. They give off a spectrum of light very similar to sunlight and are highly recommended for reef aquariums. Metal Halide bulbs require a special ballast unit for operation.

A group of soft-bodied invertebrates that includes snails, clams and squids. Most mollusks have some sort of hard external shell.


Nano Reef
A type of small marine aquarium that is typically less than 20 gallons (76 liters). Nano reefs are difficult to keep because of the small water volume. They are only recommended for experienced marine aquarium hobbyists.

NO3 the final product in the nitrogen cycle. It is not toxic, but can be dangerous at high levels. Nitrate is created by the oxidation of nitrite by nitrobacter bacteria. In a reef tank, nitrate levels should be kept below 10 ppm.

The process by which bacteria converts ammonia into nitrite and then nitrite into nitrate. This is the basis of the nitrogen cycle.

Nitrifying Bacteria
Naturally occurring bacteria that change ammonia or ammonium into nitrite or change nitrite into nitrate as part of the nitrogen cycle. Nitrifying bacteria are a key component of a biological filter for an aquarium.

NO2 the second product in the nitrogen cycle. Nitrite is a highly toxic substance that is produced by the oxidation of of ammonia by nitrosomonas bacteria. It is easily removed with biological filtration.

The bacteria in a biological filtration system that converts nitrite into nitrate.

Nitrogen Cycle
The nitrogen cycle describes how wastes are broken down by bacteria in the aquarium. Animal waste breaks down into toxic ammonia (NH3). The ammonia is oxidized by nitrosomonas bacteria into nitrite (NO2), another highly toxic substance. Another bacteria called nitrobacter oxidizes the nitrite into nitrate (NO3), a much less toxic substance. Some systems are capable of taking the process one step further, by using anaerobic bacteria to convert the nitrate into harmless nitrogen gas.

The bacteria in a biological filtration system that converts ammonia into nitrite.


A device used to continuously replace evaporated water and maintain a constant specific gravity.

The process by which a liquid passes from an area of low concentration through a semi-permeable membrane to an area of high concentration.

Osmotic Stress
An adverse reaction caused when the salinity of an animal's environment changes drastically.

Oxygen Reduction Potential (ORP)
This is simply a measurement of the water's ability to cleanse itself.

Ozone, or O3, is a very reactive form of oxygen which is commonly used in conjunction with a protein skimmer to enhance skimming and kill bacteria. Ozone must be used carefully as too much can be toxic to fish and invertebrates.

A device that uses high voltage electricity to produce ozone.


An organism that feeds on the tissues of another organism. Parasites are one of the major causes of disease in aquarium fishes.

Pectoral Fins
A pair of fins on a fish located directly behind the gills.

Pelvic Fins
A pair of fins on a fish located directly below the gills. Not all marine fishes have these.

A measure of the concentration of hydrogen and hydroxide ions. The pH of a solution measures how acidic or alkaline it is. pH values range from 0 to 14. A neutral solution has a pH of 7. A pH less than 7 indicates an acidic solution while a pH greater than 7 indicates an alkaline solution. pH can be regulated in the aquarium by using buffering materials. pH and alkalinity can also be maintained by the use of kalkwasser.

A nutrient that can case uncontrolled growth of algae in the aquarium. It can also be toxic in high concentrations and must be kept to a minimum in coral reef aquariums. Phosphate can be easily removed by a number of commercially available filter media.

The length of time each day that the aquarium lights remain on.

The process by which carbon dioxide, water, and certain inorganic salts are converted into carbohydrates by green plants, algae, and certain bacteria, using energy from the sun and chlorophyll.

Tiny microscopic plants found drifting in seawater. Phytoplankton represent the bottom of the food chain in the ocean

A generic term used to refer to both phytoplankton and zooplankton.

A small, submersible pump commonly used inside an aquarium to provide additional water movement. Several powerheads can be used in conjunction with a controller unit to simulate natural wave actions.

Protein Skimmer
An external filtering device that uses bubbles to remove nitrogen rich proteins, fatty acids, and other organic wastes. This is a required piece of equipment for maintaining good water quality in a reef tank.


Quarantine Tank
An aquarium set up separately from a main or display system used to house new livestock. Also known as a hospital tank, it is used to prevent spreading diseases into an established aquarium.


An isolated container, usually located near the sump, that performs a specific task such as increasing calcium or oxygen in the water.

Reduction-oxidation potential. It is a measure of the ability of water to allow biological reactions to take place and is used as an indication of water quality. Redox can be measured with special electronic probes, and higher readings are better.

Reef Aquarium
A saltwater marine aquarium that attempts to recreate a natural coral reef environment by using natural live rock, coral, fish, and invertebrates that are naturally found on coral reefs.

Reverse-Flow Filtration
A biological filtration system where water is returned to the tank through the base instead of through the top.

Reverse Osmosis (RO)
A process for filtering water for use in an aquarium. This method works by forcing water under pressure through a special membrane. Reverse Osmosis (R/O) can produce very pure water, but it is a slow process and can only filter small amounts at a time.


A measure of the amount of salt in seawater, measured in parts per thousand (ppt). Natural seawater has a salinity of about 35 ppt.

Silicone Sealant
A transparent, rubbery adhesive used in aquariums to bond glass and plug leaks. It can also be used in reef tanks to attach rock and coral formations.

Specific Gravity
The ratio of density of a given liquid to that of pure water. Specific gravity is used to measure the salinity of seawater as compared to distilled water. Distilled water has a specific gravity of 1.000 while natural seawater has a S.G. of about 1.025.

Sponge Filters
A type of filter that provides both mechanical and biological filtration. As water passes through the sponge, particles are removed. Bacteria growing on the surface of the sponge also remove toxic substances from the water.

A trace element found in seawater that is required for corals and creatures with calcareous skeletons to grow. Strontium levels can be maintained through regular water changes and by the use of strontium additives.

The low area of water in an aquarium system. The sump is the reservoir below the dry section of a wet/dry or trickle filter. The water level in the sump varies with evaporation.

Sweeper Tentacles
Long stinging tentacles used by some aggressive hard corals to sting other nearby corals in order to obtain territory and growing space.

Swim Bladder
A specialized organ that allows fish to maintain a chosen depth while swimming in water.

A length of tube that uses gravity to move water from one location to another. Also, the organs used by some mollusks to inhale and exhale water.

A phenomenon where two different organisms live together in a mutually beneficial relationship. Both organisms in provide each other with food, protection, or some other survival need. The most famous example is the anemone and clownfish. The anemone provides protection to the clownfish within its stinging tentacles, and the clownfish provides the anemone with scraps of food.


Trace Elements
Elements that occur naturally in small quantities in seawater. These are required for survival by many reef organisms, and include calcium, iodine, strontium, molybdenum, lithium, and barium.

Trickle Filters
A filtration system where water is dripped over some medium that is exposed to the air. The air helps to enhance the nitrification process. The filter medium usually consists of small plastic balls or strips of plastic.

The rate of water flow through a filter. A high turnover rate is recommended for reef tanks.


Ultraviolet (UV) Sterilizer
A device that sterilizes water by passing it through a glass tube around an ultraviolet light. UV sterilizers can help remove bacteria, parasites, and algae spores from aquarium water. However, they can also remove some beneficial organisms from reef tanks.

Undergravel Filter
A filtration system that provides both mechanical and biological filtration. It consists of a plate that is placed underneath the gravel. Water is pulled down through the gravel where it is filtered and carried back up into the aquarium.


A special type of valve that produces air bubbles by drawing air into a stream of water flowing under pressure. Venturi valves are used on a variety of protein skimmers.

Any animal that has a backbone and a skeleton. In the ocean, vertebrates include both jawless fish and jawed vertebrates, which includes the cartilaginous fish (sharks and rays) and bony fish.

VHO Lights
Very High Output (VHO) lights are specially designed fluorescent lights that give off a much higher intensity light than regular fluorescent bulbs. This makes them much more effective as light sources for reef systems, since many corals require strong light. As with all fluorescent lights, VHOs require a ballast unit for operation.


Water Change
The process of replacing a portion of aquarium water with a fresh saltwater mix. It is recommended that 20 to 25 percent of the water be changed each month in a reef tank.

Wet/Dry Filter
A biological filtration system that is exposed to the air to aid nitrification. This system typically consists of a large box that is placed underneath the aquarium. Water passes down into the filter over a filtration medium where bacteria remove toxins. The water is then pumped back up into the tank. A sponge or other mechanical filtration medium may also be used in a wet/dry filter.


Tiny microscopic animals found drifting in seawater. This includes the larval stages of many fishes and invertebrates.

Tiny plants that live in a symbiotic relationship with certain corals, clams, and some sponges. They receive nutrients from their host and provide a food source in return. It is the zooxanthellae that are responsible for the brilliant green, yellow, and blue colors in corals and clams.

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