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Aquatic Terminology

Actinic Lights:

Light that produces an identifiable or measurable change when it interacts with matter. Actinic lighting is visibly blue and will need a full spectrum bulb to help offset the color distortion. The light wavelength promotes the growth of Zooxanthellae algae, essential for the growth and well-being of all photosynthetic corals and invertebrates.


The adjustment to a changing aquatic environment. When marine life is transferred from one habitat or environment to another.

Activated Carbon:

Porous charcoals made with a special manufacturing process that results in highly porous charcoals that have surface areas of 300-2,000 square meters per gram. Activated Carbon is able to remove contaminants from water and air by holding the contaminants in the porous areas.


Any of numerous groups of chlorophyll-containing aquatic organisms ranging from microscopic single-celled forms to multicellular forms. Algae is distinguished from plants by the absence of true roots, stems, and leaves and by a lack of nonreproductive cells in the reproductive structures.


A measure of the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. For aquariums, alkalinity controls and maintains water pH.


Ammonia is a colorless gas with a very distinct odor. Ammonia occurs naturally and is an important source of nitrogen which is needed by plants and animals. Ammonia poisoning is one of the biggest killers of aquarium fish. It occurs most often when a tank is newly set-up. However, it can also occur in an established tank when too many new fish have been added at one time, when the filter fails due to power or mechanical failure, or if bacterial colonies die off due to the use of medications or sudden change in water conditions.


An organism that does not require oxygen for growth and even dies in its presence.

Asexual Reproduction:

Reproduction in which it is not necessary to have two parents to produce offspring. The reproduction of single-celled organisms through fission, and the production of spores in some plants and plantlike organisms, are examples of asexual reproduction. Some corals reproduce asexually.


An electrical device for starting and regulating fluorescent and discharge lamps.

Base Rock:

A generic term for aragonite rocks which have no bacterial organisms or coralline algae growing in or on the rock. Base rocks are often used to build height and depth in reef tank set-ups.

Berlin System:

A filtration technique for reef aquariums developed in Berlin. The Berlin system relies on live rock, powerful water circulation and strong skimming to filter the water.

Biological Filtration:

A method of filtration using bacteria to change toxic compounds such as Ammonia into safer compounds such as Nitrates.

BNC Connection:

(Bayonet Nut Coupling) A commonly used plug and socket for networking applications that provides a tight connection. Using a mount somewhat similar to the way a bayonet (knife) is mounted onto the end of a rifle, BNCs are used to connect a variety of different coaxial cable types. After the plug is inserted, it is turned, causing pins in the socket to be pinched into a locking groove on the plug.


Brackish water has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. Brackish water contains between 0.5 and 30 grams of salt per liter—more often expressed as 0.5 to 30 parts per thousand (ppt or %).


Corals are the main builders of a reef's framework through the accumulation of limestone by a process known as calcification. Coral calcification is intimately related to the internal symbiosis with the zooxanthellae. Calcification in corals takes place external to the organism.

Calcium Reactor:

A device used to create a balance of alkalinity in the system. An acidic solutions produced by injecting carbon dioxide into a chamber with salt water and calcium-rich media. The carbon dioxide lowers the pH, dissolving the calcium media, and producing a solution high in carboxylic acid. The effluent is then returned to the reef aquarium where the calcium is consumed by organisms, primarily corals when building skeletons. A calcium reactor is one of the most efficient methods of supplying calcium to a reef aquarium.


Any animal eating primarily meat. Most have a complex tooth structure and a lower jaw that can move only vertically but can exert great power.


A negative electrode from which electrons are emitted and to which positive ions are attracted. A component found in fluorescent lamps.


(Consumer Electronics) The CE Mark indicates conformity to the legal requirements of the European Union (EU) Directive with respect to safety, health, environment, and consumer protection. This visible declaration proves that the product complies with all applicable Directives.

Chemical Filtration:

A method of filtering using media, most often activated carbon, to remove unwanted substances in water.


Equipment that removes heat from aquarium water via a vapor-compression or absorption refrigeration cycle.

Closed Loop:

An aquarium system in which all components are connected to each other with no other opening other than the aquarium. Canister filters are an example of a closed loop filter.

Color Rendition Index:

(CRI) is a measure of the ability of a light source to reproduce the colors of various objects being lit by the source. It is a method devised by the International Commission on Illumination.

Compact Fluorescent (CF):

A type of fluorescent lamp designed into an appliance-friendly shape to increase functionality. Like all fluorescent lamps, Compact Fluorescent lamps contain mercury and should be disposed of properly. Visit to learn more about disposal.


An animal that feeds on corals. May include fish, sea stars or mollusks. There are two recognized types; obligate corallivores; those that feed only on corals and facultative corallivores, which feed on corals, algae, sponges, and mollusks.

Correlated Color Temperature:

(CCT) A characteristic of visible light, the color temperature of a light source is determined by comparing its chromaticity with a theoretical, heated black-body radiator. The Kelvin temperature at which the heated black-body radiator matches the color of the light source is that source's color temperature. Lower Kelvin numbers mean the light has a warmer color, while higher Kelvin numbers mean the light has a cooler color.


A group of arthropods, including barnacles, lobsters, crabs, shrimp and crayfish.


(Canadian Standards Association) Organization that tests products for compliance to national and international standards, and issues certification marks for qualified products. Certification marks tell potential customers and users that a product has been evaluated by a formal process-involving examination, testing and follow-up inspection-and that it complies with applicable standards for safety and performance.


The name "cyanobacteria" comes from the blue color of the bacteria, which explains why they are often referred to as "blue-green algae"; however, they are not algae. Cyanobacteria obtain their energy through photosynthesis and are a significant component of the marine nitrogen cycle.


The process of establishing bacteria in an aquarium to break-down toxic waste. The three components involved to make this happen are ammonia (NH³ or NH³+4), nitrite (NO²), and nitrate (NO³). In general the cycling process usually takes about one month; however, each aquarium is different. Factors such as how many fish, other livestock, and the amount of organic matter present in the tank can vary the cycling time.


Equipment used to remove ions from a solution. A common variant contains a mixture of cation exchange resin in the acid form and anion exchange resin in the hydroxyl form inside a replaceable cartridge; ions in aqueous solution are exchanged for the elements of water by passing the solution through the mixed resin.


The process of reducing nitrate and nitrite into gaseous nitrogen, which is less accessible to life forms. Denitrification proceeds through some combination of the following steps: nitrate › nitrite › nitric oxide › nitrous oxide › dinitrogen gas and can be expressed as a redox reaction: 2NO3- + 10e- + 12H+ › N2 + 6H2O.


Non-living particulate organic material such as dead plants, organisms or fecal matter. Detritus is normally colonized by communities of microorganisms which act to decompose the matter.


A major group of eukaryotic algae and one of the most common types of phytoplankton. Most diatoms are unicellular, although some form chains or simple colonies.


A group of flagellate protists; mostly marine plankton with populations dependent on temperature, salinity, and depth. About half of all dinoflagellates are photosynthetic, and make-up the largest group of eukaryotic algae aside from the diatoms. Dinoflagellates are an important part of the aquatic food chain. Some species, called zooxanthellae play an important part in the biology of coral reefs.

Drip Loop:

To arrange an electrical cord so it hangs down well below the outlet before running back up again. Any water running down the cord will drip down from the loop, rather than running uphill to enter the outlet.


Negatively charged particles that surround an atom's nucleus. Exciting the mercury atoms with high speed electrons in a fluorescent lamp will produce light. This is accomplished in fluorescent lamps by heating a cathode at the end(s) of the lamp, which frees some electrons. Once freed, the electrons can be accelerated in the tube by the applied voltage. Some of the electrons transfer energy to the mercury atoms in collisions, so that one or more of the electrons of the mercury atom is elevated to an excited state.

Fluidized Bed:

Aquarium fluidized bed filters involve suspending media in a body of flowing water. Fluidized bed filters are ideal for planted aquariums, as they won't dissipate the CO2.

Foam Fractionator:

See Protein Skimmer.


Female Pipe Thread, a standard connection type used to connect various parts to each other.


Ground Fault Circuit Interrupter, a device used to measure if any electrical current is "leaking" from the circuit. If a "leak" is determined the GFCI will switch the power off.


Equipment that adds heat to aquarium water to keep the tank inhabitants from falling below a particular temperature.


Any animal eating primarily plant matter. Sea urchins are a good example of a marine herbivore as they feed on kelp.


(High Intensity Discharge) HID lamps are different from standard halogen lamps in that HID lamps replace the filament of the light bulb with a capsule of gas. The light in HID lamps is emitted from an arc discharge between two electrodes. HID lamps require ballasts to operate, which supply proper voltage and control current. The amount of light produced by HID lamps is greater than a standard halogen lamp. HID lamps consume less power and last longer than halogen lamps.


An instrument used for determining the specific gravity of marine aquarium water. Specific Gravity for marine aquariums is usually between 1.021 and 1.026.


The opposition to the flow of alternating current in a circuit. Represented by the letter "Z" and measured in ohms, impedance is the combination of resistance, inductance and capacitance of the circuit.


Animals which lack a backbone. Most commonly used to describe coral, but scientifically describes snails and other shelled fish.


A trade name for calcium hydroxide. The terms "Kalkwasser," "limewater," and "calcium hydroxide" are all the same meaning.


A warm, shallow, quiet waterway separated from the open sea by a reef crest.

Lagoon Slope:

Back reef on a barrier or atoll reef.


A juvenile form of animal with indirect development, undergoing metamorphosis.

Lateral Line:

A fish's sense organ used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the gills to the base of the tail.


(Light Emitting Diode) A diode is the simplest sort of semiconductor device. The semiconductor material for LED's is typically aluminum-gallium-arsenide (AlGaAs). In pure aluminum-gallium-arsenide, all of the atoms bond perfectly to their neighbors, leaving no free electrons (negatively-charged particles) to conduct electric current. Light is produced from energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called photons, are the most basic units of light. Photons are released as a result of the moving electrons.

Live Rock:

Rock made-up of calcium carbonate skeletons of long-dead corals or other calcareous organisms which is encrusted with many forms of micro and macroscopic marine life that live on and inside of it.


Algae, such as seaweeds, that project more than one centimeter above the substratum.


A fossil coral reef.

Maximum Pumping Height:

The vertical height a pump will attain and have a flow rate equaling zero gallons per hour.

Mechanical Filtration:

A method of removing solid particles from the aquarium water.


A filter media that will allow certain molecules or ions to pass through it by diffusion. The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute.


(Symbol Hg) A silvery-white poisonous metallic element, liquid at room temperature and used in fluorescent lamps to furnish mercury vapor.

Metal Halide Lamp:

A high-pressure discharge lamp that is enclosed in a quartz tube containing metal halides; usually iodides, and produces high-efficacy light. These lamps are sometimes used in marine aquarium applications. These lamps always require a ballast, which supplies the proper voltage and current for starting and operating the lamps.


Unicellular aquatic plants (phytoplankton) and the beginning of the aquatic food chain.

Mixed Bed Resin:

Media primarily used in water purification for the polishing process to achieve demineralized water quality. Mixed Bed, as the name states, consists of strong acid cation exchange and strong base anion exchange resin that removes total dissolved solids (TDS) from membrane filtered water.


Members of the large and diverse phylum Mollusca. Mollusks live in a wide variety of habitats, in the oceans, on land and in freshwater and include clams, snails, cuttlefish, squid and octopus. There are approximately 112,000 species within this phylum and the majority of mollusks live in marine environments.


Male Pipe Thread, a standard connection type used to connect various parts to each other.


(millivolt) A unit of potential equal to one thousandth of a volt.


(nm) A unit in which wavelengths of light are expressed. 1 nm = 0.000000001 meter (1 billionth of a meter).


A polyatomic ion with the empirical formula NO³-, nitrates are the result of bacterial breakdown of ammonia and nitrite also known as the nitrogen cycle. In aquariums, nitrate can reach high levels that can potentially cause the death of fish. While less toxic than ammonia or nitrite, 30 ppm or more of nitrate can inhibit growth, impair the immune system and cause stress in some aquatic species.


The biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of these nitrites into nitrates. The oxidation of ammonia into nitrite, and the subsequent oxidation to nitrate is performed by two different nitrifying bacteria; Nitrosomonas and Nitrobacter.


The nitrite ion is NO2- and is the product of the second stage in the nitrogen cycle as bacteria break-down ammonia.


A rod-shaped bacteria that is an important part of the nitrogen cycle, oxidizing nitrite in aquarium water into nitrate.

Nitrogen Cycle:

The process of converting ammonia to less toxic nitrates in the aquarium. Ammonia (NH³ or NH³+4) is converted to nitrite (NO²) by Nitrosomonas bacteria and then Nitrobacter bacteria convert the nitrite to nitrate (NO³).


Rod-shaped chemolithoautothrophs with an aerobic metabolism. These bacteria convert ammonia with oxygen to nitrite.


An electrical resistance offered by a current-carrying element that produces a voltage drop of one volt when a current of one ampere is flowing through it. Units of ohms, kilohms and megohms are used in electronic component documentation.


An animal that eats both plants and animals as its primary food source.


(Oxidation Reduction Potential) Measured in millivolts (mV). A measurement of the loss and gain of electrons by a molecule, atom or ion. For aquarium keeping purposes, ORP is the measurement of the "livability" of the aquarium water. Natural seawater has a range of 350-400 mV; however some aquarists have broken the 400 mV barrier with great success.


The net movement of water across a semi permeable membrane from a region of low solute concentration to a solution with a high solute concentration.

Osmotic Stress:

Occurs when the concentration of molecules in solution outside of the cell is different than the concentration inside the cell. Water flows either into or out of the cell by osmosis, thereby altering the cell environment and causing stress. Hyperosmotic stress causes water to exit the cell, resulting in cell shrinkage, which can lead to DNA and protein damage and ultimately cell death. Damage to fish scales and skin can increase the susceptibility to infection and cause excessive uptake of water by freshwater fish or loss of water from marine species (osmotic stress).


Produced when oxygen (O2) molecules are dissociated by an energy source into oxygen atoms and subsequently collide with an oxygen molecule to form an unstable gas, ozone (O3), which is can be used to disinfect aquarium water. Ozone must be generated in proximity to the aquarium because it is unstable and decomposes to elemental oxygen in a short amount of time after generation. Ozone is a very strong oxidant and virucide and works by destroying cell walls.


Equipment used to generate ozone (O3) by charging the air with a burst of high negative voltage.

PAR Value:

(Photosynthetically Available/Active Radiation) The amount of light available for photosynthesis in the 400 to 700 nanometer wavelength range.


An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host.


A polyatomic ion with the empirical formula PO4, Phosphate is a prime food source of algae, particularly cyanobacteria or slime algae.


A substance that exhibits sustained glowing after exposure to energized particles such as electrons. Phosphor is used to coat the inside of fluorescent bulbs and transforms ultraviolet radiation into visible light. The type of phosphor determines the color of light output.


The duration of the daily exposure to light; considered especially with regard to the effect of the exposure on growth and development.


Autotrophic component of plankton. Most phytoplankton are too small to be individually seen with the naked eye in aquaria. However, when present in high enough numbers, phytoplankton may appear as a green discoloration of the water due to the presence of chlorophyll within their cells.


Small or microscopic organisms, including algae and protozoans, that float or drift in great numbers in aquaria. Often found at or near the surface, plankton serve as food for fish and other larger organisms.


An invertebrate having a hollow, somewhat cylindrical body, attached at one end, with a mouth surrounded by tentacles at the free end. Polyps may be solitary (hydra) or colonial (coral).

Power Factor:

A measure of the effectiveness with which an electrical device converts volt-amperes to watts.

Protein Skimmer:

Equipment used in marine aquariums to remove organic pollutants from the water before they break down. In the skimmer, tiny air bubbles attract and hold pollutants, which are then deposited into a separate container as foam.

Red Slime:

See Cyanobacteria. To prevent this potential problem from occurring, learn about what phosphate is, where it comes from, and how to reduce or eliminate the accumulation in marine aquariums.

Reverse Osmosis:

(RO) A separation process that uses pressure to force a water through a membrane that retains the solute on one side and allows the pure water to pass to the other side.

Reverse-flow Filtration:

A biological filtration system where water is returned to the aquarium at the base instead of at the water surface.


The dissolved salt content in a body of water. Typically, freshwater <500 ppm, brackish water 500-30,000 ppm, seawater 30,000 - 50,000 ppm and brine >50,000 ppm.

Sexual Reproduction:

A union that results in increasing genetic diversity of the offspring by utilizing chromosomes from two gamates.

Silicone Sealant:

A powerful, flexible product used in many marine applications to keep water in or out of a designated area. Silicone sealant remains very flexible, even once it has fully dried or cured.

Solenoid Valve:

An ectromechanical valve used to control liquid or gas flow in a pressurized line. Often used in marine and planted aquariums to control CO2 introduction into the system.

Specific Gravity:

Relative density defined as the ratio of the density of a given substance, to the density of water (H2O). Substances with a specific gravity greater than 1 are heavier than water, and those with a specific gravity of less than 1 are lighter than water. Marine aquarium water is measured in specific gravity. The specific gravity of seawater at 35 ppt is 1.026. Most marine aquariums are maintained between 1.020 to 1.026.

Spectral Distribution:

The intensity of spectral energy in wavelengths emitted by a light source.


A container in addition to the aquarium that holds additional water that flows into/out of the aquarium. Sumps may incorporate or accommodate equipment such as protein skimmers, reactors, heaters or chillers.


Term used to express the relation of one organism to another from various degrees of close relationship between organisms of different species. An example is the relationship between a clownfish and anemone.

T5 HO Lamp:

A type of high output fluorescent lamp with a 5/8" diameter. This slim profile makes T5 lamps more efficient than standard fluorescent tubes. T5 HO lamps offer many benefits; including lamp life, power efficiency, high lumen output per watt and small size. Like all fluorescent lamps, T5 HO Lamps contain mercury and should be disposed of properly. Visit to learn more about disposal.


(TDS) The combined content of all inorganic and organic substances contained in a liquid which are present in a molecular, ionized or micro-granular suspended form. The principal application of TDS is in the study of water quality.

Thermal Switch:

A safety device that interrupts electrical power when a specific temperature is reached.

Trace Elements:

Elements essential for growth, but required only in minute amounts. Trace elements for marine aquariums might include Calcium, Strontium, Iodine and other major and minor trace elements.

Trickle Filters:

Filters that may include mechanical, chemical and biological attributes and allow water to flow freely from the aquarium and over media.


The rate of water leaving and re-entering the aquarium, expressed in the count per hour that the entire water volume of the aquarium can leave and re-enter the aquarium.


(Underwriters Laboratories) Organization that tests and certifies consumer products meet specific safety criteria.

UV Sterilizer:

Equipment that sterilizes water by allowing ultraviolet light to penetrate water through a quartz glass barrier. UV Sterilizers can help remove bacteria, parasites, and algae spores from aquarium water. However, they can also remove some beneficial organisms from reef tanks.


(Ultraviolet Light) Electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. Ultraviolet light is used in aquarium applications in the germicidal wavelength — 185-254 nanometers, which renders unwanted organisms sterile. When organisms can no longer reproduce, they die.


A measure of power or the rate of energy consumption by an electrical device when it is in operation. Watts are calculated by multiplying the voltage at which an appliance operates by the current it draws (Watts = Volts X Amperes).


(for lighting) The distance between crests of a wave measured in nanometers. The wavelength determines the nature of the various forms of radiant energy that comprise the electromagnetic spectrum.

Wet/Dry Filter:

Equipment that facilitates biological filtration by allowing water to flow across a media on which Nitrifying bacteria are attached. The media is not submerged, but kept moist to facilitate the proliferation of aerobic bacteria.


Tiny, free-floating organisms in aquatic systems. Unlike phytoplankton, zooplankton cannot produce their own food, and so consume food from the aquaria.

Zooxanthellae Algae:

Tiny plants called dinoflagellates that live symbiotically with corals, tridacnid clams, and some sponges. The algae provides food for the host and in return receives the nitrogen, phosphorous, and carbon dioxide it needs to grow.


Light which produces an identifiable or measurable change when it interacts with matter. Actinic lighting is visibly blue and will need a full spectrum bulb to help offset the color distortion. The light wavelength promotes the growth of Zooxanthellae algae, essential for the growth and well-being of all photosynthetic corals and invertebrates.

10,000 Kelvin:

A spectrum that produces a full range of white light that is beneficial to Marine organisms. The lamp mimics the color of light that can be found in equatorial regions of the world.


These substances do not chemically react and therefore it's easy to control nutrient concentrations in inert media.