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Acidic: A reading below 7.0 on the pH scale.
Activated carbon: A manufactured form of carbon that is highly porous.
Adipose fin: A small fin between the dorsal and caudal fins, most notable in tetras and other characoids.
Adsorption: The way that dissolved waste binds onto the surface of a filter medium, such as activated carbon, during chemical filtration.
Aeration: Adding extra air into water to improve its oxygen content.
Aerobic bacteria: Bacteria that require oxygen to survive.
Airstone: A porous device that splits up the airflow from an air pump into small bubbles to improve water movement and oxygenation.
Algae: Primitive aquatic plants that photosynthesize.
Alkaline: A reading that is above 7.0 on the pH scale.
Anal fin: An unpaired fin on the underside of the body, near the vent.
Annual: A plant that grows, flowers, sets seed, and dies within a year.
Aquarium peat: Peat that can be added to the aquarium filter to acidify the water. Unlike garden peat, it is free from additives.


Barbels: Sensory growths around the mouths of various bottom-dwelling fish, including catfish and koi.
Biological filtration: The use of aerobic bacteria to break down waste matter in aquariums and ponds.
Blackwater: Water that has been acidified and darkened by tannin produced by decaying vegetation. Blackwater is used to encourage spawning in fish such as tetras.
Brackish water: A mix of fresh and salt water found in estuaries.
Breeding brush: An artificial spawning medium that is typically used in ponds. Fish spawn over the brush and their eggs attach to its bristles.The brush is then removed from the pond so that the eggs can be hatched elsewhere in safety.
Breeding trap: A device used to separate a gravid female livebearer from other tank occupants. It also prevents her from eating her own fry.
Brood: The offspring produced by a pair of fish; typically a group of fry that is guarded by one or both adults.
Brood pouch: A pouch on the body of male sea horses and pipefish, in which the eggs are incubated.
Bubble-nest: A nest made by many anabantoids and some catfish to protect eggs and fry. It consists of air bubbles trapped in mucus, and is usually anchored to vegetation.


Cartilage: A tough, flexible body tissue. In some fish, the skeleton is made entirely of cartilage.
Caudal fin: The tail fin, which is often divided into lobes.
Caudal peduncle: The muscular shaft that links the body of a fish to the tail (caudal) fin.
Chemical filtration: The use of chemicals to remove dissolved waste by adsorption.
Chromosome: A gene-carrying structure found in the nucleus of every living cell. Genes determine the characteristics of all organisms.
Classification: A method of grouping living things to show the relationships between them.
Community tank: An aquarium that houses a number of different yet compatible species.
Compatibility: The degree to which fish interact favorably with one another; also, the bonding of a pair of fish prior to spawning.
Conditioning: Managing the fish and their surroundings to encourage breeding. Conditioning also refers to the way in which water is treated to make it safe for the tank occupants.
Coral sand: Crushed coral, with a particle size similar to that of sand. Coral sand can be added to the filter to make the water more alkaline.
Crown: The central area of a plant, from which new growth develops.
Cultivar: A cultivated example of a plant that does not occur in the wild. The common name is often suffixed by “cv,” meaning “cultivated variety.”
Cutting: Part of a plant that is removed and used for propagation.


Dechlorinator: A chemical preparation that removes chlorinebased compounds from tap water.
Dorsal fin: The unpaired fin that runs along the center of the back. Some fish have a divided dorsal fin.


Ectothermic: Describes an animal whose internal body temperature varies according to its surroundings.
Egg spots: Egg-shaped markings on the anal fin of some male cichlids. As mouth-brooding females nibble at the egg spots, they take in sperm released by the male, ensuring that the eggs in their mouth become fertilized.
Egg-layer: Any fish in which the eggs are fertilized and hatched outside the body.
Environment: A living thing’s surroundings. For a fish, this includes the water, the substrate, and the plants and animals with which it interacts.
Evolution: The origin of species by development from earlier forms.


Family: A group of related genera.
Fancy: Describes fish that have been bred to emphasize ornamental qualities, such as coloration or fin shape, that are not seen in wild forms.
Filter bed: A layer of substrate through which water passes during the filtration process.
Filter feeder: An invertebrate that feeds by sifting tiny food particles from water.
Filter medium: Any material used in a filtration system to remove waste, or which can be colonized by beneficial aerobic bacteria.
Filtration: The removal of waste from water in aquariums and ponds.
Fins: The projections on a fish’s body that enable it to move through water. They are also often used for display and even mating purposes.
Flake: Manufactured fish food in the form of thin, waferlike fragments that float at the surface. Suitable for both freshwater and marine fish.
Flexible liner: A sheet of butyl rubber or PVC that is used to form a watertight lining for a pond.
Flow adjuster: A valve that regulates the movement of water through a pump.
Frost-hardy: Describes a plant that is able to survive frost unprotected, but not extreme cold.
Fry: Newly hatched or newborn fish.


Genetics: The branch of science that deals with the way characteristics pass from one generation to the next.
Genital pore: An opening on the underside of the body, marking the entrance to the genital tract.
Genus (plural genera): A group of closely related species.
Gills: The main respiratory organs of a fish, located on each side of the head behind and below the eyes. Gills extract dissolved oxygen from water.
Gonopodium: The modified anal fin of male livebearers, which is used for mating purposes.
Gravid: Describes a female whose body is swollen with eggs or developing young.


Habitat: The place where a plant or animal naturally lives.
Half-hardy: Describes a plant that is likely to be killed off by frost if left unprotected.
Hand-stripping: The manual removal of eggs from a female fish and sperm from a male.
Hard water: Fresh water containing a high level of dissolved calcium and magnesium mineral salts, typically in excess of 150 milligrams per liter.
Hardy: Describes a plant that can withstand regular exposure to freezing winter temperatures.
Hermaphrodite: An animal with both male and female sexual organs.
Hospital tank: A small, simply equipped tank that allows sick fish to be treated and recover in isolation from those in the main aquarium.
Hybridization: The cross-breeding of different species together.
Hydrometer: A device used to measure the specific gravity—and hence the salinity—of water.


Invertebrates: Animals without a backbone (vertebral column).
Ion-exchange column: A watersoftening device that uses resins to remove mineral salts from tap water.
Isolation tank: A tank in which new fish are quarantined before being added to the main aquarium, to make sure that they are free from disease.


Labyrinth organs: Auxiliary respiratory organs, located close to the gills, that enable anabantoids to breathe air at the water’s surface.
Larva (plural larvae): The posthatching stage in an invertebrate’s lifecycle. Marine fish fry are often called larvae, since they are poorly developed when they hatch.
Lateral line: A fluid-filled canal that runs horizontally along each side of a fish’s body. It contains sensory pores that detect pressure changes in the water, aiding navigation and giving early warning of impending danger.
Length: A standard dimension for describing fish, taken from the snout to the end of the caudal peduncle, but not including the caudal fin.
Live rock: Marine rock that harbors many different invertebrate and plant organisms, some of which will not be apparent to the naked eye.
Livebearer: A fish whose eggs are fertilized and develop inside the body.
Livefoods: Invertebrates used as fish food.They can be bred at home and fed to the fish alive, or purchased in frozen or freeze-dried form.


Marginals: Plants that can be grown in shallow water around the pond edge (includes so-called bog plants).
Mechanical filtration: A method of filtration that relies on straining particulate matter out of the water.
Metabolism: The biochemical processes that occur in living things.
Midline: The central horizontal axis of a fish’s body.
Morph: A naturally occurring color variant of a species.
Mouth-brooder: An egg-laying fish that incubates and hatches its eggs in the mouth. Some cichlids and anabantoids are mouth-brooders.
Mulm: Waste matter, such as fish feces, unwanted food, and decaying plant debris, that builds up on the floor of the aquarium or pond.
Mutation: An unexpected change in the genetic makeup of an organism.


Nauplius (plural nauplii): The larval stage in the life cycle of the brine shrimp, which is an important rearing food for fry.
New tank syndrome: The sudden illness and/or death of fish that occurs when inefficient filtration allows toxic chemicals such as nitrite and ammonia to build up in the water.
Nitrogen cycle: The natural process by which nitrogenous waste is recycled in aquatic environments. Ammonia excreted by fish is broken down by bacteria, first into nitrite and subsequently into nitrate. Plants absorb the nitrate and use it for growth. Fish then eat the plants, completing the cycle.
Nuchal hump: The swelling on the forehead of mature male cichlids.


Offset: A new plant that develops when a runner sets down roots and detaches from the parent plant.
Operculum (plural opercula): The covering over the gills, sometimes called the gill flap.
Ovipositor: An extendable tube used by some egg-layers to deposit their eggs at breeding times.
Oxygenators: Plants that mostly grow below the surface and give off streams of oxygen bubbles in sunlight.


Parasite: An organism that lives on or in the body of a host animal or plant, and feeds off it.
Pectoral fins: Paired fins, one on each side of the body behind the gills.
Pelvic fins: Paired fins on the underside of the body in front of the anal fin.
PH: An expression of the hydrogen ion content of water.The pH scale runs from 1 to 14, with a pH of 7 being neutral. Low numbers indicate acidity, and higher numbers alkalinity.
Pharyngeal teeth: Projections in the throat of cyprinids and some other fish, which help to break down food.
Photosynthesis: The process by which plants use light energy to make food from carbon dioxide and water, releasing oxygen as a waste product.
Plankton: Microscopic plant and animal life found in the sea. Plankton provides a rich source of nourishment for many marine creatures.
Power filter: A self-contained filtration system that incorporates a motorized pump to move water through the filter.
Powerhead: A small pump that creates surface water movement to improve aeration.
Prepared foods: Foods that have been specifically formulated to meet the nutritional needs of fish.
Protein skimmer: A filtration device that creates and collects an electrically charged foam, which draws waste products from the water.


Rays: The bony supports found in the fins of many different types of fish.
Reverse osmosis (RO): A process used by some water-softening units, in which water is forced through a membrane to remove mineral salts.
Rhizome: A swollen plant stem that spreads underground and produces shoots along its length, which then develop above ground.
Rotifer: A component of plankton, used by fishkeepers as a first food for rearing marine fish.
Runner: A creeping horizontal stem that grows above the ground, and on which new plants (offsets) develop.


Salinity: The concentration of dissolved mineral salts, especially sodium chloride, in water.
Scales: Small protective platelets that cover the bodies of most fish.
School: A group of fish that associate together, usually (but not always) of the same species.
Scute: A scale modified into a bony plate, found especially in some catfish.
Sessile: Attached to a surface.
Sexual dimorphism: A difference in the appearance of the sexes. In fish this may include differences in size, color, patterning, or fin shape.
Shoal: A group of fish of the same species that swims together.
Siphon: A tube for removing water from an aquarium; also, a tube through which water enters and leaves the body of mussels and other invertebrates.
Soft water: Fresh water containing less than 100 milligrams per liter of dissolved calcium and magnesium mineral salts.
Spawning: The process of egglaying and fertilization.
Spawning mop: Strands of synthetic yarn attached to a float, on which fish can be persuaded to spawn.
Spawning pit: An area of substrate excavated by some species of fish, in which they spawn or guard their fry.
Species: A group of animals or plants with similar characteristics that can breed together in the wild to produce fertile offspring.
Specific gravity (SG): The density of a liquid containing dissolved minerals compared to that of pure water, which has an SG of 1.00. SG can be used to measure salinity.
Strain: A selectively bred form that has distinctive characteristics.
Subspecies: A distinct population within a species.
Substrate: The material on the floor of an aquarium, pond, river, or lake.
Swim bladder: A fish’s buoyancy organ. It may also produce sounds.
Symbiosis: A beneficial relationship between two different species.


Taxonomy: The study of the naming and classification of living things.
Territorial: The readiness of an animal to defend a particular area.
Trace element: Minerals, such as iron, that an organism needs in small amounts to ensure its well-being.
Tube feet: Projections on the underside of starfish and some other marine invertebrates, which propel the animal over the sea floor.
Tuber: The swollen storage organ of a plant, which normally grows at least partially underground.
Tubercles: White swellings seen on male cyprinids prior to spawning.


Undergravel filter: A filtration system in which a perforated plate sits under the gravel substrate.This allows oxygenated water to flow through the gravel, promoting the growth there of aerobic bacteria, which break down waste matter.


Variety: Another word for a strain.
Vent: The anogenital opening, which is located close to the anal fin in fish.
Ventral fins: An alternative name for the pelvic fins.
Vertebrate: An animal with a backbone (vertebral column).


Water conditioner: A preparation that makes tap water habitable for fish. It combines a dechlorinator with other ingredients, such as aloe vera.


Yolk sac: The part of an egg that nourishes a young fish before and immediately after hatching.


Zeolite: A clay-based compound that removes ammonia from water.
Zooxanthellae: Single-celled algae that live inside animals such as corals in a symbiotic relationship. The algae grow in the relative safety of the coral’s body, and provide food for the coral when they photosynthesize.