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An Aquarist’s Glossary of Terms – Part 2

Check out the 1st part of Eileen’s Aquarist’s Glossary of Terms here.

Water chemistry terms:


  •  Alkalinity: Alkalinity is often confused with pH or water hardness. In technical terms, the alkalinity is the ability of a solution to neutralize acid. In an aquarium, this can be seen in changes in pH. The higher the alkalinity, the more difficult it is for the pH to change. pH levels above neutral (7.0 on a scale of 0-14) are also said to be “alkaline” or “basic” as opposed to levels below neutral are considered acidic.
  • Brackish: Brackish environments are those found in between freshwater and saltwater environments and have a specific gravity of about 1.005 to 1.015 or a salinity of 0.5-30 ppt (parts per thousand). Common brackish water environments related to the aquarium trade are mangrove swamps, estuaries and bays.
  • Carbon Dioxide: Carbon dioxide (CO2) is a compound used by plants to produce oxygen in photosynthesis and a byproduct of respiration in which oxygen is used. CO2 levels are important to monitor in heavily planted freshwater aquariums but are not as important in unplanted freshwater aquariums or in saltwater systems.
  • Freshwater: Freshwater is water that does not have a significant salt level and is typically of most river or lake environments. Common freshwater environments that many aquarium-trade fish can be found in are the the Amazon River, African Rift Lakes, Australian rivers and South and Central American rivers. These are also referred to as aquatic environments.
  • Nitrogen Cycle: The Nitrogen Cycle is used to descripe the process of converting ammonia (NH4+) to nitrite(NO2) to nitrate (NO3). This process happens in any environment with organisms that produce ammonia and is conducted by nitrifying bacteria (nitrosomonas and nitrobacter). The levels of these three compounds are some of the most universal and important levels to monitor in home aquariums as they can be toxic to aquarium animals. Ammonia and nitrite can be especially toxic as they affect the ability of an animal’s gills to function correctly and the ability of an animial to absorb oxygen into its bloodstream. This is also known as the “Cycling Process” of an aquarium and can take 4-6 weeks in new aquariums or aquariums in which the biological filtration has been destroyed by water changes, medications or another cause.
  • Ozone: Ozone (O3) is a molecule made up of three oxygen atoms instead of the more stable 2-atom molecule (O2). In aquariums and ponds, ozone is used to help control bacteria, disease and algae growth, to help remove very tiny debris particles from the water and for several other purposes. An “ozonizer” is used to pump a controlled amount of ozne into the aquarium. This is safe for most aquarium species (some animals like sharks can be very sensitive to ozone) but can quickly dry out and crack some rubber materials like tubing. Silicone tubing should be used with ozone and ozonizers.
  • pH: The pH level is, technically speaking, the amount of free hydrogen ions in a substance. The pH level is calculated using mathematical logarithms and does not have a unit of measurement. This scale is measured from 0 to 14 (or -1 to 14 in some scientific circles) with 7.0 being considered “neutral”. Levels above 7.0 are considered “Basic” or “alkaline” while levels under 7.0 are considered “acidic”. Some common pH ranges in the aquarium trade are 8.0-8.4 for marine systems, 6.8-7.4 for most tropical freshwater fish like those from the Amazon River, and 7.8-8.4 for African Cichlids from Lake Malawi and Lake Victoria.
  • Phosphate: Phosphate (PO4) has many uses, from glass production and fertilizers to matches and fireworks. An excess of this nutrient in freshwater or saltwater systems can cause algae blooms, usually characterized by green water or greenish or blackish films on surfaces. It can be removed by special filters or filter media and usually enters an aquarium through overfeeding with phosphate-rich foods or though source water with high phosphate levels like well water or in heavily agricultural areas.
  • Salinity/ Specific Gravity: Both of these terms are used to describe and measure the amount of salt in water. “Salinity” is mainly used in more scientific measurements and measures the amount of salt in parts per thousand (ppt or 0/00). “Specific gravity” is more common in the aquarium trade and measures the density of a substance compared to pure water (since this “measurement” is actually a comparison, it does not have a unit of measure like “ppt” in salinity). Salinity is commonly measured using weight, chemical tests or refraction (the water’s ability to bend light waves); Specific gravity is measured using a hydrometer (a device that measures buoyancy to determine density). Marine aquariums usually have a salinity of 30-35ppt or a specific gravity of around 1.020-1.025
  • Saltwater: Saltwater environments are those with a specific gravity of 1.018 or higher, or with a salinity of 30 ppt (parts per thousand) or higher. This is the environment found in oceans and seas around the world and is home to coral reefs. The term “marine” is also used to refer to anything pertaining to these environments.
  • Water Hardness: Water Hardness is a measure of the amount of minerals dissolved in the water, especially Calcium and Magnesium. Like alkalinity, the water hardness affects the stability of water – the chemistry of soft water is much easier to change than hard water. Some fish like Discus or most tetras prefer soft water while others like African cichlids and Bettas thrive in hard water. Working with the hardness of your water when choosing fish and maintaining an aquarium can be far less stressful than trying to change it. Water hardness is usually measured two ways – General Hardness and Carbonate Hardness. Both are usually measured using liquid test kits and a method called “titration” – adding small amounts of solvent to a solution until and endpoint like a color change is reach.
  • General Hardness is usually measures in degrees (dGH or ºGH). 1 dGH is equal to 10 milligrams of calcium oxide per liter of water or 17.848 ppm. “Very soft” water is defined as 0.4 dGH, “Soft water” is 4-8 dGH, “Slightly Hard” water is 8-12 dGH, “Moderately hard” is 12-18 dGH, and “Hard” water is 18 dGH or higher.

The Carbonate Hardness (KH) is closely related to the General Hardness but is a measure only of the calcium carbonate (CaCO3), not of the other minerals present. This is usually more important in saltwater aquariums than in freshwater since most saltwater invertebrates have skeletons or shells made up of primarily calcium carbonate. The GH and KH are usually close but can be different, depending on the minerals present.

An Aquarist’s Glossary of Terms

Hi, Eileen here. There are a lot of different terms and phrases, used in reference to saltwater and freshwater aquariums alike, that may be unfamiliar to some aquarists or that you might come across in books, magazines or your local fish store like That Fish Place/ That Pet Place. I thought it might be beneficial to place a glossary of common terms as a reference. Here are some of the most common terms used in the aquarium trade that may be confusing to someone just starting out or beginning to explore the hobby. Keep in mind that they may have different meanings or may be used differently depending on the region or the aquarist you may be speaking to.

Biological terms:

  • Barbel: the “whiskers” or whisker-like appendages around the face and mouth of some fish.
  • Benthic: Benthic organisms live on, in, or attached to the bottom. These organisms include corals, crabs, starfish, cucumbers and worms.
  • Breeding styles: There are several different breeding styles common in aquarium fish and invertebrates.
    • Livebearer: Livebearers give birth to live young that are fully or nearly fully formed and often resemble miniature adults. Guppies, swordtails and seahorses are a few well-known livebearers.
    • Mouthbrooder: Mouthbrooders hold their eggs and eventually their young in their mouths until the young are ready to live on their own. The parent holding the young often does not eat during this time. Some cichlids and saltwater cardinalfish are mouthbrooders.
    • Egglayer: This is the breeding style that most people are familiar with. The female fish will lay a clutch of eggs and the male will fertilize the eggs after they are released. Most parents will guard their eggs during this time. Freshwater angelfish and saltwater clownfish are common egg-layers.
    • Eggscatterer: Egg-scatterers show little to no parental care for their young and will randomly scatterer the eggs across the substrate or vegetation. They often may eat their eggs and fry if not removed from the site. Tetras, danios, and rasboras are all egg-scatterers.
    • Bubblenesting: Gouramis and bettas are well-known bubble-nesters. The male will build a ring or nest of bubbles at the surface of the tank or plants and will defend other fish, including the female who laid the eggs, from getting close to the next.
    • Eggburying: Annual fish like killifish will bury their eggs in muddy substrate in the wild where they remain dormant throughout the dry season and until the next rainy season. These fish are typically short-lived and only have a lifespan of one or two seasons.
  • Cichlid: Cichlids (pronounced SICK-lids) are a group of fish known for being some of the most aggressive and territorial. They are native to distinct regions and many species have limited range. Cichlids are generally divided into “Old World” and “New World” groups.
    • Old World Cichlids: These cichlids are those found generally in the eastern hemisphere, most notably the African Cichlids found in the African Rift Lakes Lake Malawi, Lake Tanganyika and Lake Victoria basin. This group also includes West African, Madagascar and Asian cichlids. These are typically found in very specific water conditions and should not usually be mixed with other more docile tropical fish.
    • New World Cichlids: These are also collectively referred to as South and Central American cichlids but may also include some found in southern North America. Many are large and aggressive, but there are also dwarf species and some that are rather docile.
  • Coral: Coral refers to a very large group of invertebrates in the family Cnidaria. They are very widespread in appearance, habitat, requirements and lifestyle, but are only found in saltwater environments. This group can further be dividing into LPS, SPS, and Soft Corals.
    • Large Polyp Stony Corals (LPS): These corals have hard skeletons covered with fleshy tissue that often can make the coral look like a plant or anemone. These corals are found in a variety of environments and are often not as sensitive as their SPS cousins. They feed through a combination of photosynthesis from algae in their tissue and filter feeding from the water column. This is not technically a scientific classification, but is used by hobbyists as a convenient way of dividing corals.
    • Small Polyp Stony Corals (SPS): These corals have a more rigid appearance with very small polyps or thin tissue covering their skeletons. They live primarily on reefs and most get the majority of their nutrition from the algae in their tissue. These corals are very popular amongst hobbyists for propogation and “fragging”. Again, like LPS corals, this is not an official scientific classification.
    • Soft Corals: Also known as leather corals, the vast majority of these do not have a rigid skeleton, though there are a few exceptions, like Heliopora. They filter-feed or absorb most of their nutrition from the water and can live in a wide range of water conditions.
  • Crustaceans: Crustaceans are a group of invertebrates characterized by their insect-like appearance, hard exoskeleton that they must “molt” or shed to grow, and segmented bodies. Common aquarium crustaceans are shrimp, crabs and lobsters.
  • Cyanobacteria: Cyanobacteria (“cyano” or “slime algae”) is a bacteria commonly mistaken for algae and can be green, brown, red, black or blue. It is considered an aquarium nuisance and usually will bloom when water quality or lighting is poor. It is most common in saltwater aquariums, but can also be found in freshwater aquariums at times.
  • Diet: Diet refers to what an animal will normally eat in the wild. Five main diet or feeding types are common in aquarium:
    • Herbivore: Herbivores eat plants and plant matter almost exclusively.
    • Carnivore: Carnivores are meat-eaters and feed on very little plant matter.
    • Omnivore: Omnivores eat both plants and animals in their nature diet. Some may eat more plants than meat or vice versa.
    • Planktivore: Plantkivores feed on very small plants and animals in the water column. They may actively search out this food or may filter feed by sifting through the water column and feeding on whatever they find there.
  • Photosynthesis: Some invertebrates have algae known as zooxanthallae living in their tissue which produces proteins through photosynthesis. The animal then feeds off of the by-product of these symbiotic algae.
    • Diurnal: Diurnal organisms are active primarily during the day or in daylight.
  • Fin types: Each fin and body region on a fish is named and the common or scientific name of the fish may refer to markings on or around these fins. Knowing the names of the fins can also help in identifying fish or diseases. Not all of these fins are found on every fish and there are several other minor fins found in some groups.
    • Dorsal: The dorsal fin runs along the back and spine of the fish. The dorsal side of the fish also refers to this top half or the region around its back. This fin is used to help stabilize the fish while swimming.
    • Pectoral: Pectoral refers to the chest of the fish or the fins on either side of the body just behind the gills. These fins are used mostly in steering the fish, but also help to propel it through the water.
    • Peduncal: The peduncal or caudal peduncle is the narrow area between the main body of the fish and the tail.
    • Pelvic: Pelvic fins are paired and are found under the pectoral fins on the lower part or “belly” of the fish. They are used to help stop or turn the fish and to move vertically through the water column. Some fish like gobies have fused pelvic fins that act like a suction cup and help fix the fish onto a surface.
    • Anal: This fin is located on the bottom of the fish just in front of the tail. It is also used to help stabilize the fish. In male livebearers, this fin is modified into the pointed gonopodium and is used to fertilize the female.
  • Invertebrate: An invertebrate is any animal without a backbone. This group includes snails, shrimp, crabs, corals, octopuses, clams, starfish, urchins and many more animals.
  • Lateral Line: The Lateral Line is a sensory organ found primarily in fish. The Lateral Line can be seen as a groove running the length of the body on most fish and is used to detect movement in the water, even from great distances. Schooling behaviors and the quick movement and reflexes seen in some fish are thought to be related to this organ. Some fish can be identified by markings around the Lateral Line and some diseases like Head and Lateral Line Erosion (HLLE) are associated with it as well.
  • Nematocyst: Nematocysts are stinging cells found in the invertebrate phylum Cnidaria (anemones, jellyfish, corals and other invertebrates). The cell is “spring-loaded” – when the trigger is activated, the cell springs open and injects a harpoon-like projection into the target. This is a defense in most animals but some fish like jellyfish use this as a passive way to catch and disable their prey.
  • Nocturnal: Nocturnal organisms are active primarily at night.
  • Operculum: Operculum is Latin for “little lid”. In fish, this is the bony plate that covers the gills. In snails, this is the hard trapdoor that protects that snail when it retracts into its shell.
  • Plankton: Plankton and planktonic organisms do not swim against the flow of the water but rather travel with the flow. Most plankton is very tiny and is a primary food of filter-feeding organisms like corals and feather dusters, but other planktonic organisms like jellyfish and even the very large Ocean Sunfish can be much larger in size.
  • Photoperiod: The photoperiod is the length of day and night an organisms is exposed to and their reaction to it. An improper photoperiod can affect the health and activity of aquarium fish, invertebrates and plants.
  • Photosynthesis: Photosynthesis is the process by which a plant (including algae) uses carbon dioxide and sunlight to produce organics like sugars and gives off oxygen as a by-product. Some bacteria like cyanobacteria also use photosynthesis.
  • Scales: Most fish are covered with rigid scales for protection. These scales vary with the type of fish. Fish like sharks have scales also known as “dermal denticles” for their tooth-like structure. Other fish have scales with either a smooth outer edge (cycloid), or scales with a serrated edge (ctenoid).
  • Scientific name: The scientific name of an organism is the name assigned to it by scientists and taxonomists to be used as the “official” and universal name for that organism. In contrast, the common name can vary between countries, languages, regions and from aquarist to aquarists. Scientific names are typically written in italics while common names are often written with quotation marks.
  • Sexual Dimorphism: Sexually dimorphic animals have physical characteristics that differentiate between males and females of the same species. The difference can be subtle (a specific marking or size difference) or very obvious (completely different coloration).
  • Venomous: A venomous organism injects a toxin into its prey, either as a defense or as a way to disable their prey. This differs from poisonous organisms whose toxin must be ingested or absorbed.
  • Zooxanthallae: These tiny algae cells live within the tissue of other organisms like corals and anemones. The larger animals or colony provides the zooxanthallae with a host and the zooxanthallae provides the host animal with energy from photosynthesis. A coral may “bleach” or eject all of the zooxanthallae from its tissue if stressed, often resulting in the death of the coral.

I’ll be back with some other terms that may be useful for those who are new to the hobby in future posts!