Marine Aquarium Glossary

The process of gradually adjusting a new marine organism to the water conditions in your aquarium. Typically involves slowly mixing tank water into the bag or container over 30 to 60 minutes to prevent osmotic shock from sudden changes in temperature, salinity, and pH.

A measure of the water's buffering capacity, or its ability to resist changes in pH. In marine aquariums, alkalinity is primarily maintained by bicarbonate and carbonate ions. The ideal range for reef tanks is 8 to 12 dKH. Stable alkalinity is critical for coral growth and skeletal formation.

A highly toxic nitrogen compound (NH3/NH4+) produced by fish waste, uneaten food, and decaying organic matter. It is the first compound in the nitrogen cycle and must be converted to nitrite by beneficial bacteria. Any detectable level of ammonia is dangerous to marine life.

Refers to biological processes or zones that occur without oxygen. In marine aquariums, anaerobic zones within deep sand beds or porous live rock harbor denitrifying bacteria that convert nitrate into harmless nitrogen gas, completing the nitrogen cycle.

An automated system that detects when aquarium water levels drop due to evaporation and replenishes with fresh RO/DI water to maintain stable salinity. ATO systems use float switches or optical sensors and prevent the salinity spikes that occur when water evaporates and salt concentration increases.

The use of beneficial bacteria colonies to break down toxic ammonia into nitrite and then into less harmful nitrate through the nitrogen cycle. Live rock, bio-media, and sand beds provide surface area for these bacteria to colonize and thrive.

A device that dissolves calcium carbonate media using CO2-infused water to maintain calcium and alkalinity levels in reef aquariums. Essential for tanks with heavy SPS coral loads that consume large amounts of calcium for skeletal growth.

The practice of adding an organic carbon source (such as vodka, vinegar, or commercial products) to promote the growth of bacteria that consume nitrate and phosphate. This biological method helps reduce excess nutrients that can fuel nuisance algae.

Tiny crustaceans that form a vital part of the marine food web. In aquariums, copepods reproduce in refugiums and live rock, serving as a natural food source for fish such as mandarin dragonets, wrasses, and seahorses. Many reef keepers seed their tanks with cultured copepod populations.

The process of cutting or breaking a piece from a parent coral colony to create a new fragment (frag) that can be mounted and grown independently. Fragging is used for propagation, trading, and controlling coral growth within an aquarium.

A type of encrusting red algae that forms pink, purple, and magenta calcified coatings on live rock, glass, and equipment. Coralline algae is a sign of a healthy, mature aquarium with good calcium and alkalinity levels. It competes with nuisance algae for space and nutrients.

The process of establishing beneficial bacterial colonies in a new aquarium to handle the nitrogen cycle. During cycling, ammonia levels rise, then nitrite, and finally nitrate, as different bacterial populations become established. A fully cycled tank shows zero ammonia and zero nitrite.

Accumulated organic waste material found on substrate, rocks, and in filter media. Composed of fish waste, uneaten food, dead organisms, and other decomposing matter. Excessive detritus buildup can degrade water quality by releasing nitrate and phosphate.

The practice of briefly immersing new corals in a pest-removal solution before placing them in the display tank. Coral dips help eliminate hitchhiking pests such as flatworms, nudibranchs, red bugs, and unwanted crabs that can damage or consume coral tissue.

An automated peristaltic or diaphragm pump that delivers precise, measured amounts of supplements (such as calcium, alkalinity, and magnesium solutions) to an aquarium on a programmable schedule. Essential for maintaining stable water chemistry in reef tanks.

A sand bed of 4 to 6 inches or more in depth that creates anaerobic zones in the lower layers. These oxygen-free zones foster denitrifying bacteria that convert nitrate into nitrogen gas, providing natural nitrate reduction in the aquarium.

A common name for tube-dwelling polychaete worms of the family Sabellidae. They extend a fan-shaped crown of feathery radioles from a parchment-like tube to filter feed on plankton and particulate matter. Often found hitchhiking on live rock.

The volume of water moved through pumps, powerheads, or filtration systems per unit of time, typically measured in gallons per hour (GPH) or liters per hour (LPH). Proper flow rate ensures adequate gas exchange, waste removal, and nutrient delivery to corals.

Short for fragment. A small piece of coral that has been cut or broken from a larger colony for the purpose of propagation or trade. Frags are typically mounted on small ceramic or rock plugs and grown out in dedicated frag tanks or racks.

A phosphate-removing filter media made from iron oxide granules. GFO is placed in a media reactor or filter bag where it chemically binds dissolved phosphate from the water column. It is one of the most effective methods for controlling phosphate levels in reef aquariums.

A unit of measurement for water flow rate used to rate pumps, powerheads, and filtration equipment. A general guideline is to turn over the total tank volume 10 to 20 times per hour for reef aquariums to ensure adequate circulation.

Short for metal halide. A type of high-intensity discharge lighting once considered the gold standard for reef aquariums. Metal halides produce intense, full-spectrum light that effectively penetrates deep water columns, though they generate significant heat and consume more energy than modern LEDs.

A type of filter or equipment designed to hang on the back rim of an aquarium. HOB filters, protein skimmers, and refugiums are popular for smaller tanks or setups without a sump, offering easy installation and maintenance access.

A basic instrument used to measure the specific gravity (and thus salinity) of aquarium water. While inexpensive and easy to use, hydrometers are less accurate than refractometers and should be calibrated regularly for reliable readings.

Marine ich, caused by the parasite Cryptocaryon irritans, is one of the most common diseases in saltwater aquariums. It presents as white spots on the fish's body and fins, along with scratching, rapid breathing, and lethargy. Treatment typically involves copper-based medications or tank transfer method in a quarantine tank.

A saturated solution of calcium hydroxide (lime water) used to supplement calcium and alkalinity in reef aquariums. Traditionally dripped into the tank or added to the auto top-off system, kalkwasser also helps maintain elevated pH and can precipitate phosphate.

Light Emitting Diode. The current standard lighting technology for marine aquariums, offering energy efficiency, programmable spectrums, and long lifespan. Modern reef LEDs can produce the high PAR values needed for demanding SPS corals while allowing precise control over color temperature and photoperiod.

Porous aragonite rock harvested from the ocean or aquacultured, colonized with beneficial bacteria, coralline algae, and diverse micro and macro organisms. Live rock serves as the primary biological filtration in most marine aquariums and provides natural structure for fish to hide and corals to attach.

Aragonite sand that has been seeded with beneficial bacteria and micro-organisms. It provides biological filtration, a natural substrate for burrowing organisms like gobies and jawfish, and contributes to the buffering capacity of the aquarium water.

A classification of stony corals characterized by large, fleshy polyps and a calcium carbonate skeleton. LPS corals such as Euphyllia, Acanthastrea, and brain corals are generally hardier than SPS corals and tolerate moderate lighting and flow, making them popular choices for intermediate reef keepers.

Large, visible forms of marine algae such as Chaetomorpha, Caulerpa, and Gracilaria. Grown in refugiums or display tanks, macro algae absorb excess nitrate and phosphate from the water as they grow. Regularly harvesting the algae exports these nutrients from the system.

An essential element in marine aquarium water that helps maintain proper calcium and alkalinity balance. The ideal range is 1250 to 1350 ppm. Low magnesium makes it difficult to maintain calcium and alkalinity levels and can inhibit coralline algae and coral growth.

Common name for dragonets of the genus Synchiropus, particularly the Mandarin Dragonet (Synchiropus splendidus). Known for their vivid coloration and slow, hovering movement. Mandarins are specialized feeders that primarily consume live copepods, requiring a mature tank with a large copepod population or training onto prepared foods.

Physical removal of suspended particulate matter from aquarium water using filter pads, socks, sponges, or roller mats. Mechanical filtration traps debris before it decomposes and degrades water quality. Filter media should be cleaned or replaced regularly to prevent it from becoming a nitrate source.

A small reef aquarium, typically under 30 gallons. Nano reefs are popular for their compact size and affordability but require diligent maintenance since small water volumes are less stable and more sensitive to parameter swings. Species selection must account for limited swimming space.

A nitrogen compound (NO3) that is the end product of aerobic biological filtration. While far less toxic than ammonia or nitrite, elevated nitrate levels above 20 ppm can stress fish, inhibit coral growth, and promote nuisance algae. Controlled through water changes, refugiums, and carbon dosing.

A toxic nitrogen compound (NO2) produced as an intermediate step in the nitrogen cycle when Nitrosomonas bacteria convert ammonia. Nitrite is then further converted to nitrate by Nitrospira bacteria. Like ammonia, any detectable level of nitrite is harmful to marine organisms.

The biological process by which beneficial bacteria convert toxic ammonia to nitrite and then to relatively harmless nitrate. Establishing this cycle is the fundamental first step in setting up any marine aquarium and typically takes 4 to 8 weeks to complete.

A measurement in millivolts (mV) of the water's ability to oxidize contaminants and break down waste. Higher ORP values generally indicate cleaner, more oxygen-rich water. Typical target range for marine aquariums is 300 to 450 mV. Protein skimmers and ozone generators can raise ORP.

A device or built-in weir system that allows surface water to drain from the display tank into a sump below. Overflows skim the water surface to remove proteins and oils, improve gas exchange, and maintain a consistent water level in the display aquarium.

A measurement of light intensity in the wavelengths usable by photosynthetic organisms (400 to 700 nm). PAR values help reef keepers determine whether their lighting is adequate for different types of corals. SPS corals typically require PAR of 200 to 400+, while soft corals thrive at 50 to 150.

A type of positive-displacement pump that moves fluid by compressing flexible tubing with a rotating roller mechanism. Widely used in marine aquariums for precise chemical dosing, auto top-off systems, and calcium reactors due to their accuracy and self-priming capability.

A logarithmic scale measuring the acidity or alkalinity of water, ranging from 0 (most acidic) to 14 (most alkaline). Marine aquariums should maintain a stable pH between 8.1 and 8.4. pH naturally fluctuates throughout the day due to photosynthesis and respiration cycles.

A nutrient compound (PO4) that enters the aquarium through fish food, tap water, and decaying organic matter. Elevated phosphate levels above 0.03 ppm can fuel nuisance algae growth, inhibit coral calcification, and degrade overall water quality. Controlled with GFO media, water changes, and refugium algae export.

The duration and schedule of light exposure in an aquarium over a 24-hour period. A typical marine aquarium photoperiod is 8 to 10 hours of full lighting, with gradual ramp-up and ramp-down periods to simulate natural sunrise and sunset conditions.

Microscopic organisms that drift in ocean and aquarium water. Phytoplankton are plant-like organisms that photosynthesize, while zooplankton are tiny animals that feed on phytoplankton or other zooplankton. Both serve as essential food sources for filter-feeding corals, clams, and certain fish species.

The individual living unit of a coral colony. Each polyp is a small, soft-bodied organism with a central mouth surrounded by tentacles used for feeding. In stony corals, polyps secrete a calcium carbonate skeleton. The health and extension of polyps is a key indicator of coral well-being.

A submersible water pump used to create circulation and flow within the aquarium. Proper placement of powerheads eliminates dead spots, ensures even nutrient distribution, and simulates natural ocean currents that corals and fish require. Many modern powerheads offer controllable, wave-making patterns.

A filtration device that removes dissolved organic compounds from aquarium water before they decompose. It works by injecting fine air bubbles into a reaction chamber where organic molecules adhere to bubble surfaces and are collected in a waste cup as dark skimmate. Considered essential equipment for most marine aquariums.

A separate, bare-bottom tank used to isolate and observe new fish for 4 to 6 weeks before introducing them to the main display aquarium. Quarantine tanks allow for disease treatment with copper or other medications that would harm corals and invertebrates in a reef tank.

A separate, connected chamber or tank that provides a protected environment for growing macro algae, copepods, and other beneficial organisms. Refugiums help reduce nitrate and phosphate through algae nutrient export and serve as a breeding ground for natural fish food.

A multi-stage water purification system that removes chlorine, chloramine, heavy metals, phosphate, silicate, and other impurities from tap water. RO/DI water is considered essential for mixing saltwater and topping off evaporated water to prevent introducing contaminants into the aquarium.

The concentration of dissolved salts in aquarium water, measured in parts per thousand (ppt) or specific gravity (sg). Natural seawater has a salinity of approximately 35 ppt or 1.025 sg. Maintaining stable salinity is critical, as even small fluctuations can stress marine organisms.

The ratio of the density of saltwater to the density of pure water, used as a practical measurement of salinity. The ideal specific gravity for marine aquariums is 1.023 to 1.026. Measured using a hydrometer or refractometer, with refractometers being more accurate.

A classification of stony corals with very small polyps and dense calcium carbonate skeletons. SPS corals such as Acropora, Montipora, and Stylophora demand high light intensity, strong water flow, and pristine, stable water chemistry. They are considered the most challenging corals to keep successfully.

A secondary tank or reservoir, typically located beneath the display aquarium, that houses filtration equipment such as protein skimmers, heaters, return pumps, and refugium sections. Sumps increase total water volume for greater stability and keep equipment out of the display tank for a cleaner appearance.

A measurement in parts per million (ppm) of all dissolved substances in water, used primarily to test the output quality of RO/DI systems. A TDS reading of 0 ppm from the RO/DI unit indicates the membrane and resin are functioning properly and producing pure water.

Minor and micro elements found in natural seawater that are consumed by corals and other organisms. These include iodine, strontium, iron, manganese, and others. Trace elements are replenished through regular water changes or targeted supplementation to support coral health and coloration.

A device that passes aquarium water over an ultraviolet light bulb to kill or disable free-floating bacteria, parasites, and algae spores. UV sterilizers help control disease outbreaks and reduce green water algae blooms. They do not replace quarantine but add an extra layer of protection.

The practice of removing a portion of aquarium water and replacing it with fresh, pre-mixed saltwater. Regular water changes of 10 to 20 percent weekly dilute accumulated nitrate, phosphate, and other waste products while replenishing trace elements and minerals consumed by tank inhabitants.

Symbiotic dinoflagellate algae (genus Symbiodinium) that live within the tissues of most reef-building corals, clams, and some anemones. Zooxanthellae photosynthesize and provide up to 90 percent of their host's energy needs in exchange for shelter and nutrients. Coral bleaching occurs when these algae are expelled due to stress.