Glossary of Terms: Understanding Marine & Reef Aquarium Terms

Glossary of Terms


The marine and reef aquarium hobby can be a bit overwhelming when you first start out. The fish and coral are AMAZING, but what the heck is Acclimation and do you need a Calcium Reactor? We hope this glossary of terms and acronyms helps you out.

This glossary was written to help you navigate through some reef aquarium terms and acronyms. Seriously, there are enough terms, concepts, and acronyms to fill an entire book, but in this glossary, we will just cover some of the most commonly used terms and to provide information that will help guide you in this incredible hobby.

Glossary of Terms & Acronyms


  • Acclimation – Acclimation is the process of helping an animal gently adjust to changes in its environment, such as changes in lighting, water temperature, salinity, and pH. Fish, invertebrates, and coral are very sensitive to even minor changes in these parameters, so acclimation is important. See a detailed acclimation guide here: Acclimation Guide
  • Airline Tube – Flexible tubing used to connect airstones and air-driven devices to an aquarium pump. They are frequently also used for the acclimation process as well. There are many accessory pieces you may need including T-connectors, splitters, shut-off valves and check valves.
  • Airstone – Normally a piece of limewood or porous stone, airstones are used to diffuse air into the tank, eliminating the noise and large bubbles of conventional air filtration systems.
  • Algae Bloom – An algae bloom, marine bloom or water bloom is a rapid increase in the population of algae in an aquatic system.
  • Aquaculture – The breeding and cultivation of fish, invertebrates, and corals for commercial purposes. Many aquarium fish and invertebrates can now be bred in captivity and coral is grown from fragments or “frags” in aquarium facilities, which eliminates the need to collect coral from the wild. This process significantly reduces the negative impact the aquarium hobby can have on natural ocean reefs.
  • Auto Top Off – A system used to automate the addition of water to your saltwater aquarium that is lost due to evaporation.


  • Calcium Reactor – A device that helps automate the process of replenishing calcium as well as other minerals and trace elements in a marine aquarium. Providing enough calcium to meet the demands of all the corals, invertebrates, and algae in a closed ecosystem can be a challenge for aquarium hobbyists. Aquarium water is pumped through this chamber filled with aragonite (crushed ancient hard coral) along with pressurized carbon dioxide (CO2). The CO2 lowers the pH in the chamber to an acidic level, which dissolves aragonite into the aquarium water.
  • Captive Conditioned – Fish that are sold as captive conditioned have been acclimated to the aquarium setting and are used to eating a captive diet. These fish tend to adjust better when moved to a new tank vs. fish that were recently taken from the wild.
  • Clean-up Crew – A group of invertebrates, typically including marine snails, crabs, and shrimp that are introduced to a tank to help tackle common algae issues in an aquarium. They can help control or eliminate hair algae, cyanobacteria, diatoms, film algae, and waste.
  • Compatibility Chart – Like any other animal, fish, corals, and invertebrates can be territorial or aggressive with each other. Additionally, depending on size, species, and appetites, you need to do your research before adding animals to your tank. For example, make sure you don’t buy a fish that will eat all of your coral. Additionally, some species are not tolerant of fish of the same species, unless you introduce a fish of the opposite gender. See our chart here: Compatability Chart
  • Cycling – Nitrogen cycling is a process in which the water goes through several biological changes that result in it being chemically balanced to sustain life. This process can take up to a few weeks to complete, but it’s a necessary process to keep your marine life healthy and alive.


  • dKH – The measurement of alkalinity. dKH stands for “degrees of hardness.” Alkalinity serves two major roles in a saltwater tank, first is maintaining the pH (or acidity) of the tank. Second is providing carbonate and bicarbonate, which is the basis for how corals stay healthy and grow.


  • Filter Media – Anything placed in a filter that changes the quality of water flowing through it. The three types of filter media used for marine aquariums are mechanical, biological and chemical.
  • Filter Sock – A felt filter bag used in saltwater tanks to filter the water and keep the water column free of particles so your water looks crystal clear.
  • FOWLR – Stands for Fish Only With Live Rock. FOWLR aquariums contain fish and live rock, but no living corals or invertebrates that fish may eat. Invertebrates that can be part of a FOWLR system include snails, stars, crabs, urchins, and lobsters. A FOWLR setup has an advantage over a fish-only (FO) aquarium in that the live rock in a FOWLR system adds biological filtration. In addition, FOWLR setups are typically less expensive, easier to keep, and less demanding than coral reef aquariums.


  • Hydrometer – A tool used to measure the specific gravity of the saltwater in an aquarium. Specific gravity in a saltwater aquarium is the ratio of saltwater to pure water. This method of measurement is easier and less expensive to measure than the actual salinity, which requires special equipment to measure saltwater salinity in parts per thousand (ppt). See also Refractometer.


  • Ich – Saltwater ich (ick) or “white spot disease” is one of the most common diseases that infect marine fish in home aquariums. It is caused by Cryptocaryon irritans, a parasite, which can reproduce rapidly in a relatively small closed environment like an aquarium.
  • Invertebrates – Marine invertebrates included in the saltwater aquarium hobby include animals such as crabs, snails, shrimp, anemones, clams, scallops, sea stars, and urchins. Invertebrates, also known as inverts, are the pillar of any marine tank – they are an integral part of keeping your tank clean and healthy.


  • Lighting – Aquarium lighting is a critical element of aquarium health because it provides vital energy to photosynthetic plants and animals. The most popular types of lighting available include LED (newest), high-intensity metal halide and fluorescent.
  • Live Rock – Rock made up of calcium carbonate skeletons of long dead corals or other organisms that is populated with many forms of micro and macroscopic marine life that live on and inside of it. Frequently this is coral that has broken off, died and become inhabited with numerous organisms. Live rock becomes the main biological filter in a reef tank. Note: This rock must be cycled when introduced into an aquarium. See Cycling.
  • Live Sand – Live sand is natural reef coral sand that is collected from the ocean or non-living coral sand that is cultured to make it live. What makes it live is the microscopic biological bacteria that grows on it, and the many tiny crustaceans and other micro and macro-organisms that reside in it. Live sand can serve as the main base for biological filtration in a saltwater aquarium, while the organisms help consume organic matter in the sand bed.
  • LPS – Large polyp stony coral. LPS are hard corals which have small skeletons and with large amounts of tissue that grows on top. They inflate with water to give them shape, require less light and water flow, and use their large polyps to trap food, like zooplankton. LPS are typically a bit easier to maintain in the aquarium setting and some popular examples include brain, bubble, hammer, frogspawn, blatos, acans, chalice, donut, and candy cane corals.


  • MAC Certified – The Marine Aquarium Council (MAC) provides voluntary standards and an eco-labeling system for the marine aquarium trade. The MAC works with fisheries, industry operators, and stakeholders to develop certification programs. Their programs have helped to achieve direct and indirect social, conservation, and economic benefits in source countries while, at the same time, increasing the number of certified organisms channeled to hobbyists worldwide.
  • Mariculture – A form of aquaculture, mariculture involves the growth of fish or corals in farms in the ocean vs. in an aquarium. Coral fragments or “frags” are grown in shallow lagoons or tropical seas, which eliminates the need to collect coral from the wild. This process significantly reduces the negative impact the aquarium hobby can have on natural ocean reefs.


  • Nano Tank – A nano tank typically refers to a small aquarium. There is little consistency on what defines a small aquarium, different sources suggest a nano tank may be anywhere between 10 to 30 gallons. These types of tanks are very popular in the hobby right now but do require special attention as even small changes in water quality and temperature can negatively impact the inhabitants.


  • Overflow – The overflow is simply how water gets from the aquarium to some sort of external filter, usually a sump below/behind the tank. There are internal and external overflows depending on the style of your tank. An overflow also helps keep the level of water in your aquarium steady.


  • Powerhead – A powerhead is a water pump completely submerged into an aquarium to circulate water; usually this is used to create the flow throughout the tank.
  • Protein Skimmer – A device used to remove organic compounds such as food and waste particles from water. They are used in both public and home aquariums.
  • Pump – All aquariums require one or more pumps. A return pump moves water to or from the filtration system. Circulation pumps are used to create currents in your tank. Other pumps are used to push water through devices like protein skimmers and UV sterilizers.


  • Quarantine Tank – A tank that is set up independently from an established or main saltwater aquarium and used to “quarantine” any new fish an aquarist has acquired, before introducing them into the main aquarium. May also be used to isolate a fish that is stressed or may be ill.


  • Refractometer – A device used to measure the salinity gravity and specific gravity of water. Specific gravity in a saltwater aquarium is the ratio of saltwater to pure water. A refractometer is more accurate, but a bit more expensive than a hydrometer. See also Hydrometer.
  • Refugium – A small aquarium that either sits next to or near an existing saltwater or reef aquarium. It is used to continually cycle water into and out of the existing tank. It typically contains macroalgae, sand or mud bed, and sometimes rock. A refugium recreates, on a small scale, the seagrass beds that are commonly found in and around natural reefs. It both filters the main tank’s water and provides a natural supply of foods, including phytoplankton and zooplankton.
  • Regions – Tropical fish, coral, and invertebrates come from various regions throughout the world. Unfortunately, many vendors use different region names to identify where their stock is imported from. You can view the map of the regions and the countries that are included in them that Aqua Locker uses at Regions.
  • RODI (or RO/DI) – Reverse Osmosis Deionized – A type of water purification in which tap water is run through a semipermeable membrane to remove ions, molecules, and larger particles. Most tap water contains elements which are not in natural seawater and which can inhibit the health of marine animals.


  • Salinity – The concentration of salt in aquarium water. More specifically, salinity is a measure of the dissolved sodium ions in the water and it can be measured in parts per thousand (ppt). Most saltwater aquarists measure the salinity in their aquarium using a hydrometer.
  • SPS – Small polyp stony coral. SPS are hard corals and are the reef builders of the ocean. Some examples include staghorn, which grows in branches, and velvet coral, which grows into large plates and discs. These corals put down calcium carbonate skeletons as they grow and require lots of light for energy and strong water flow to bring them food and wash away waste.
  • Sump – A sump is simply a container to collect water from a plumbed system. The sump sits below or behind the main tank and is used as a filter, as well as a holding place for unsightly, miscellaneous equipment such as protein skimmers, calcium reactors, and heaters.
  • Symbiotic Relationship – Any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasitic. These types of relationships are common in the ocean and aquarium setting, the most well-known being that between a clownfish and sea anemone.


  • TDS – Total dissolved solids. A measure of the combined content of all organic and inorganic substances contained in the water in molecular, ionized or micro-granular suspended form.
  • Testing Kit – Water quality is critical in the reef aquarium hobby. Fish, coral, and invertebrates require certain water parameters in order survive. Fluctuations in key water parameters can result in animals becoming sick, algae blooms, and a variety of other problems. For a fish-only saltwater aquarium, you should regularly test pH, Alkalinity, Ammonia, Nitrite, and Nitrate.
  • Trace Elements – The inhabitants of almost any saltwater aquarium require supplements added to the water. The trace elements of special concern for reef tanks include calcium, iodine, and strontium. Trace elements can be replenished through feeding, water changes, and liquid supplements.


  • UV Sterilizer – These devices are used in conjunction with your primary filtration system and are designed to help with water clarity and the management of water-borne microorganisms like free-floating algae.


  • WYSIWYG – What You See Is What You Get. When buying online, you frequently get to see an image of what a typical coral or fish looks like, but not the actual item you are buying. With WYSIWYG, the photos you see online are the exact item, not a representation of the type of animal you are buying.


We would like to thank the Saltwater Keepers & Reef Aquarium Facebook page for their excellent input on this glossary of terms and acronyms for this article. Feel free to send us other terms you would like to see as we will be adding a permanent Glossary of Terms to the website. Suggestions can be sent to [email protected]

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