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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.

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About Eileen Daub

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Marine Biologist/Aquatic Husbandry Manager I was one of those kids who said "I want to be a marine biologist when I grow up!"....except then I actually became one. After a brief time at the United States Coast Guard Academy, I graduated from Coastal Carolina University in Myrtle Beach, South Carolina in 2004. Since then, I've been a marine biologist at That Fish Place - That Pet Place, along with a Fish Room supervisor, copywriter, livestock inventory controller, livestock mail-order supervisor and other duties here and there. I also spent eight seasons as a professional actress with the Pennsylvania Renaissance Faire and in other local roles. If that isn't bad enough, I'm a proud Crazy Hockey Fan (go Flyers and go Hershey Bears!).