Go to Home

Coupons & Deals
Aquarium Fish Stress and Disease
Stress & Disease

Fish Deals
on EBAY!
Breeding Aquarium Fish
Aquarium Fish Compatibility





Find the best fish and fishy deals on the products you should buy.

FishDeals.com | Common Terms
Commonly Used Aquarium Terms


If you know of any commonly used aquarium terms not listed or any corrections that need to be made please send them to Ask An Expert


The process of slowly, over a period of time, introducing a fish to new water or tank conditions.

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

A variety of sour, water-soluble compounds derived by a special partial exchange of replaceable hydrogen; a substance in which an aqueous solution can form positively charged hydrogen ions (H+). Acid colors turn litmus paper red.

Activated Carbon
A form of carbon specially formulated for aquatic filtration. Activated Carbon is good for removing a large number of toxins and other unwanted substances from aquarium water. it is especially useful for clearing and "polishing" cloudy water. The only real problem with carbon is that is can release phosphate into the water, which stimulates algae growth. (IE. Dark Tank Syndrome)

Aged Water
Tap water that has been left to stand, and from which chlorine has been neutralized or water from a healthy, well-established aquarium that does not contain large amounts of waste or nutrients.

There are thousands of different variations, Any of a wide variety of photosynthetic organisms lacking a vascular system. They can be unicellular, filamentous, or complex (seaweed).

As 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 or ups and downs. Alkalinity can be raised by adding any type of carbonate buffer material. Alkalinity can also be maintained through the use of substance called kalkwasser.

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

Any labyrinth type fish; they have an special organ in the top of there head that allows them to intake atmospheric oxygen and transpose that into the blood stream.

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

Aquasafe, which is the superior Water Conditioner that we recommend. AquaSafe is available everywhere. We don't use much water conditioner, and we think it's probably over-used in many aquariums, but it's economical and convenient to use it to make safe water, and it's wise to have enough water conditioner on hand to deal with cloudy or foamy water or a disaster.

Asexual Reproduction
Asexual means having no sex or sex organs, therefore asexual reproduction would be reproducing by means other than sex.

A calcium-containing material or miniral usually found in the form of rock, sand, or gravel.


Small single celled organisms from the Moneran kingdom. They are known as prokaryotes, which are classified together because they lack nuclear membranes. They are the most primitive living beings, and help in the nitrogen cycle to reduce ammonia in the water.

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

A strong alkali which, in an aqueous solution can form negatively charged hydroxide ions (OH-). Changes litmus color blue.

Berlin System
One method of biological filtration that uses only live rock and a powerful protein skimmer.

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

A type of filter media used for the colonization of bacteria. Ususally look like a small blue ping pong ball.

Bio load
The waste demand placed upon the life-support system as a result of the metabolism of all the organisms that live in the environment or tank.

A rotating pleated structure over which water is directed as part of a biological filtration system allowing good nitrifying bacteria grow.

A 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 lighter.

Brackish Water
Water that is neither fresh water nor saltwater, but is somewhere in between. In nature this occurs at the mouths of rivers and swamps near saltwarer. Some fish live in a variety of water types, some in salt water but are spawned in brackish or fresh water and vice versa.

Brine Shrimp
A tiny species of shrimp growing to only about 0.25 inch. Also known as "Sea Monkeys", they are 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.

Bubble nest
A term used for a nest which is constructed of small air bubbles, produced by the male fish. They are used to protect the the eggs and fry. Members of the family Anabantidae are the most widely known users of such nests.

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

Bulk Head Fitting
A Plastic fitting that goes through your aquarium to connect to various part of equipment outside your aquarium.

Byssus Gland
The structure in clams that produces fibrous threads (byssus) that attach the clam to substrate. Sometimes permanent although more usually temporary attachment of tough organic threads secreted from a gland in the foot of the mollusk.


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

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

The major building block and mineral 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 Hydroxide
Ca(OH)2 a substantive 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 contanier or cannister that contains various mechanical filtration media. Water is pumped from the tank, forced through the cannister, and then returned to the tank.

A substance used mainly for filtration. See activated carbon.

Species of several families with elongated sensory barbells, a scaleless skin (sometimes with bony plates), usually a bottom-dwelling creature.

Positively charged particles: (examples Na+, Ca2+, Mg2+). In an electrical field cations seek the negative pole or negative cathode.

Caudal fin
A single fin at the back of a fish. The tail fin.

Any fish in the family Characidae, some commonly known as tetras.

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

Any unit used to lower the waters temperature. Heat is generated from different sources, pump, lighting, ambient room temperature, check to see what your species optimum temperature range is.

A substance used in municipal water supplies that kills bacteria. Chlorine is toxic to all 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.

Some municipal water treatment and storage facilities use chloramine to treat water. It is easily removed with many commercially available additives; this is toxic to your fish. Always use a declorinator agent when adding new untreated water.

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

Coralline Algae
An encrusting form of algae that forms calcareous crusts similar to 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 pH, alkalinity and calcium levels.

Hard-shelled invertebrates that includes crabs and shrimps.

Organisms that can form large colored mat like materials. They are usually blueish, greenish in color.

Marine Ick; Cryptocaryon irritans is a parasitic infection where small white spots appear on the body and fins. Fish often scratch themselves against rocks and breathing may become rapid if gills are affected. Treatment can be done by copper or other anti-parasite remedies, but this is incompatible with inverts. Cleaner shrimps and wrasses will remove the parasites, but may not keep up with a major infestation. Cryptocaryon is often referred to as the marine equivalent of the freshwater white spot disease, Ichthyophthirius, or Ick.


Common water flea, often cultivated as a type of food for fish.

An organism that commonly forms a brown slime or 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 device for filtering water that uses several ion exchange resins to purify and remove impurities from the source water.

Grayish piles of organic compounds that accumulate in an aquarium. Commonly will contain fish wastes, fragments of rock, leftover food, among other things. Usually detritus will accumulate in low water flow areas, sumps, under ornaments, etc.

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

Fruit flys, often used as Wingless or stump-winged fruit flies cultured for live fish food.

Dorsal Fin
The fin located on the very 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 add small amounts of chemicals and trace elements into the aquarium water. It is recommended that kalkwasser be dosed in this manner.


Any item 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
Any type of organism that filters out nutrients such as plankton, bacteria, or detritus from the water.

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

Fish behaviour shown by rapid, rubbing or glancing contact with an object in an effort to displace an external parasite or irritation. "Flashing" refers to the fact that the lighter underbelly is seen momentarily.

Fluidizing Bed
A method of biological filtration where water is forced through a cylinder containing small beads. Nitrifying bacteria live and grow on the beads that remove waste materials from the water.

Foam Factionation
A method of removing proteins from water using a type of foam. This is the filtration method used by protein skimmers.

Free Spawners
This term means that females drop their eggs into the seawater, and fertilization/development occur in the water column or on the tank floor.

Newly hatched eggs or born fish.


A scientific order of taxonomy which contains the names of the species.

The respiratory organs used by fish. It allows dissolved oxygen to be extracted from the water in which the fish swim.

Particular to the Live-bearing fish, the gonopodium is the pelvic fin of the male fish that have been converted into genital organs.

Gravid spot
A dark but transparent area under the base of the penduncle which female guppies possess; visible in very young female fry.


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

A device used to warm 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.

Any 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.

Head and Lateral Line Erosion
Known as hole-in-head disease and lateral line disease. A fish with this condition will develop holes in its' head and sometimes along its' lateral line. The main cause is nutritional deficiency, especially vitamin C. Stress and poor water quality also play a role. Untreated cases will cause disfiguring or death. To combat and cure, ensure good water quality and provide vitamin enriched foods, especially vitamin C.

HO lighting
High Output fluorescent lighting, very good for some tanks.

A freshwater hydrozoan resembling a small sea anemone, an aquarium pest that is sometimes a problem in breeding because it preys on fry.

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

Hydrogen Sulfide
Anerobic. A molecule composed of one hydrogen and one sulfur atom. It is a toxic compound which has a rotten egg odor. It is synthesized anaerobically by unwanted bacteria.


Ick: see cryptocaryon. Small white spots on a fish. Sometimes causing flashing.

An electrically operated propeller that causes water to flow through a pump or filter. Powerheads and Power Filters often use these.

Internal Filter
Any filtration unit which is kept inside the aquarium. Sponge Filters.

Animals without a backbone. This group includes mollusks, crustaceans, worms, corals, and composes a large number of reef inhabitants.


A term referring to water with dissolved calcium hydroxide. A kalkwasser is used to add inorganic calcium to the water.

A type of food, any euphausid crustacean (Euphausia superba) from the sea, sold frozen or freeze-dried as fish food.


The single most trait that distinguishes the Labyrinth fish from all others, is the organ they posses that gives them their name. The Labyrinth is located above the gills and consists of skin folds, called Lamelli, which are filled with blood vessels, through which oxygen can be absorbed from the air. This feature allows the Labyrinth fish to survive in water with very low oxygen levels. The general body shape of the fish varies from elongated, with slight lateral compression (bettas) to leaf shaped.

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

Lateral Line
The LL is a line of perforated scales along the flanks of a fish which lead to a pressure-sensitive nervous system. This enables the fish to detect vibrations in the surrounding water caused by other fish and their own reflected vibrations against obstacles.

Meaning pertaining to the edge of the lake, near the shore.

A fish which gives direct birth to live fry, non egg layers.

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


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.

The common name for rock dwelling African Cichlids from lake Malawi Africa.

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

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

To most people, a minnow is a small fish - any small fish. Not so. A minnow is a member of a distinct group. It has scales and bones and internal organs that distinguish it from fishes of other families. And not all minnows are small. The Colorado squawfish is a minnow. It can weigh eighty pounds. The carp is a minnow. It can weigh sixty. Of course, most minnows are small.

Metal Halide
A type of light bulb that uses special gases to give off a very bright pure 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.

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


NO3 the end 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 to nitrate. This is the basis of the nitrogen cycle.

NO2 the 2nd 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.

Bacteria which oxidatively transform nitrite into nitrate.

Bacteria which oxidatively transform ammonia or amonium to nitrite.

Nitrogen Cycle
Ammonia is created by urea and decomposition. Ammonia is turned into nitites by nitrosomonas bacteria. Nitrites are less harmful than bacteria, but still pose a threat. Nitrites are converted to nitrates by nitrobacter. Nitrates are much less toxic and is used as fertilizer for live plants. Harmful in reef tanks. There are special denitrifying filters which convert nitrates to nitrogen gas, which is explosive in high quantities.

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


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

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

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

Overall Hardness
Water hardness as produced by calcium and magnesium salts as measured by the cations Ca2+ and Mg2+.

Oxygen Reduction Potential (ORP)
A measurement of the water's ability to cleanse itself.

O3 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 03 or ozone.


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

Peat Moss
Peat is a type of moss which is used to soften water and to decrease the pH.

Pectoral Fins
A set of fins on a fish located directly rear of the gills.

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

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

The period of time that an aquariums lights remain on. For your basic Aquarium, it is recommended to only have the light on for 6 hours each day. Reef Tanks have seperate specifications depending on the life in the tank.

These are tiny microscopic plants found drifting in seawater.

A term used to refer to both phytoplankton and zooplankton. Found mainly in oceans.

A small submersible water pump commonly used inside an aquarium to provide additional flow or 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 or foam 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.

Parts per billion, equal to micrograms per litre.

Parts per million, equal to milligrams per litre.

Parts per thousand, equal to grams per litre.


Quarantine Tank - Hospital Tank
A hospital tank set up for newly acquired or sick fish to isolate them from the main aquarium until they can be safely introduced. Holds fish temporarily apart from others for assessment.


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

Reduction-oxidation potential. 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.

Reverse-flow Filtration
Biological filtration system where water is returned to the tank through the base instead of through the top.

R.O. Reverse Osmosis
A purification method for tap-water. Prefiltered tap water is pushed through a reverse osmosis membrane. Water that makes it through is considered pure, while water that does not, is sent through a special tube and is rendered impure. As it relies on water which is able to pass through the membrane, it also generates a large quantity of "waste" water which cannot be used. This is one of the best, but slowest methods of tap water purification and will increase your water bill. Reverse Osmosis units produce purified water at extremely slowly, sometimes as low as 10 or 15 gpd (gallons per day). Some fish require this water type.


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

Sea Water
Water with a salt content of around 3% (30 o/oo) or more. The chief component of the dissolved salts is sodium chloride NaCl which is, literally, household salt.

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

Softer Water
The importance of this for fish-keepers is that those fish for which we want “softer” water really do not want “soft water”, they want water with low total dissolved solids (TDS) which would include low GH (GH is measuring the concentration of calcium and magnesium ions) as we measure it, but also low Na+ and Cl-.

A taxonomical name. Every living creature is assigned a unique species name, which is composed of two parts.

Specific Gravity
A 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 & 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 variety of a certain type species. The freshwater guppy, for example, has only one species name but several strains.

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 aquatic 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.

A specialized organ that allows fish to maintain a chosen depth in water. A ballow type organ usually near or under the backbone.

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

A phenomenon where 2 seperate 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.


Trickle Filters
A filtration system where water is slowly 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.

Turnover rate
The number of times the volumn of water in an aquarium is passed through the filtration system in one hour. The minimum should be three or four times per hour.


Ultraviolet (UV) Sterilizer
This 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 Filters
This is 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 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.

Ventral fins
A set of fins on the lower part of the fish, located below the gill covers. Not all fish have these. The pelvic fin.

VHO Lights
Very High Output (VHO) lights are specially designed fluorescent lights that give off a 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.


The Basis of any aquarium, thank god for water: a liquid which, at normal temperature, commonly has no smell. It solidifies into ice at 0° C or 32° F; its boiling point is 100° C or 210° F. Consists of two atoms of hydrogen and one of oxygen (H2O).

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

Wet/dry Filter
A biological filtration system 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.


Yolk sac
A small container connected to the baby fish after hatching, consisting of the unabsorbed egg yolk.


The barred pattern or zebra-type pattern of 2 to 5 vertical dark pigmented stripes on the peduncle area of the fancy guppy, expressed only in males which is carried by a dominant gene.

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

Small 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.


    FishDeals.com Special Features  

Breeding Aquarium Fish
Compatible Groups for Aquariums
View Aquarium Coupons & Deals