Glossary
Technical terms listed alphabetically. Each entry includes a definition and context of use.
A
Acclimation: The process of gradually adapting a fish or invertebrate to the conditions of a new aquarium to prevent osmotic shock.
Aerobic: A process that requires oxygen. Nitrifying bacteria (Nitrosomonas, Nitrobacter) are aerobic.
Allelopathy: The ability of some plants to release chemicals that inhibit the growth of nearby plants.
Alkalinity: See KH. In marine aquariums, it is measured in meq/L or dKH. The water's ability to resist pH shifts toward the acidic side.
Coral algae (CCA): A pink/purple calcareous algae that colonizes rocks in a marine aquarium. A sign of good water quality. Promotes coral settlement.
Ammonia (NH3): A toxic nitrogen-containing waste product. The first stage of the nitrogen cycle. More toxic at high pH and high temperatures.
Ammonium (NH4+): The ionized form of ammonia. Much less toxic. The NH3/NH4+ equilibrium depends on the pH.
Anaerobic: A process that occurs without oxygen. Denitrifying bacteria (which convert NO3 to N2) are anaerobic and require O2-free zones in the filter.
Anemone: A marine animal (Actiniaria) that lives in symbiosis with clownfish. Its tentacles are stinging to humans. It requires stable conditions and high-quality water.
Aquascaping: The art of designing aquariums using plants, rocks, driftwood, and other elements to create an underwater garden.
ATO (Auto Top Off): An automatic system that replenishes evaporated water with fresh water to maintain a stable salinity level in seawater.
B
Nitrifying bacteria: Bacteria that oxidize toxic nitrogen compounds (NH3 → NO2 → NO3). They form the biological filter.
Biotope: An aquarium that faithfully reproduces a specific real-world aquatic ecosystem, complete with the species and conditions found in that region.
Bacterial bloom: A sudden proliferation of bacteria in the water that makes it milky. This is normal in new aquariums. It clears up on its own.
Balling (system): A method for dosing Ca, KH, and Mg in marine aquariums using separate solutions dosed daily.
Biofilm: A film of microorganisms (bacteria, algae, protists) that colonizes the surfaces of the aquarium. An essential food source for shrimp.
C
Ca (Calcium): An essential mineral for the skeletons of corals and invertebrates. Target range for marine aquariums: 380–450 ppm.
Calcium carbonate (CaCO₃): A compound that forms the skeletons of corals and the shells of mollusks. It is also found in limestone.
Caridina: A genus of freshwater shrimp. They require soft, slightly acidic water. They are more demanding than Neocaridina.
Cichlids (Cichlidae): A large and highly diverse family of fish. Ranging from dwarf species (Apistogramma) to large predators (Cichla). Many species exhibit complex parental care.
CO2 (carbon dioxide): An essential nutrient for plant photosynthesis. It is injected into planted aquariums. In excess, it is toxic to fish.
Corallimorpharia: An order of marine animals that resemble corals but lack a calcareous skeleton. It includes "mushrooms" and "fungi."
CCA: See Coralina Algae.
Cryptocaryon irritans: A ciliate parasite that causes marine ich. It is more resistant and has a longer life cycle than freshwater ich.
Quarantine: A period of isolation for new species before introducing them to the main aquarium. It allows for the detection and treatment of diseases.
Cycling: The aquarium's maturation process: establishing the biological filter through the nitrogen cycle.
D
Daphnia: A small freshwater crustacean. A nutritious live or frozen food source. Its chitinous cuticle aids digestion.
Relative density: The ratio of the mass of seawater to that of pure water at the same temperature. The ocean has a relative density of ~1.025. It is measured using a refractometer or densimeter.
Denitrification: An anaerobic process in which bacteria reduce NO3 to gaseous N2 (an inert gas). It occurs in oxygen-free zones of the filter or in nitrate reactors.
DI (deionization): A water purification process that uses ion-exchange resins. It removes all ions from the water. It is used after reverse osmosis.
Discus (Symphysodon): Highly prized cichlids. They require soft, acidic water and high temperatures (28–32°C).
DSM (Dry Start Method): An aquascaping technique that involves setting up the layout without water and keeping it moist until the plant carpet is established.
Drop checker: A container with a pH indicator solution that is placed inside the aquarium to visually monitor CO2 levels based on color.
E
Equatorial/Tropical: Refers to temperature and light conditions that simulate those of the tropics (24–28°C, 10–12 hours of light).
Ectoderm: The outer layer of coral tissue. Zooxanthellae live in the gastroderm (middle layer).
Emerged: The part of the plant or aquarium that is above the water level.
Skimmer / Protein Skimmer: Marine aquarium equipment that removes organic matter using air bubbles.
F
Biological filter: The colony of nitrifying bacteria that converts NH3 into NO2 and then into NO3. They live in the porous filter media.
Fishless cycling: A method of cycling an aquarium without fish, by adding pure ammonia or food as a nutrient source for the bacteria.
Photoperiod: The duration of the lighting cycle in the aquarium. Generally 8–10 hours for tropical fish.
Photosynthesis: The process by which plants and corals (through zooxanthellae) convert light and CO2 into sugar and O2.
FOWLR (Fish Only With Live Rock): A type of marine aquarium that contains only fish and live rock, with no corals.
Frag: A piece of coral separated from the parent colony for reproduction or exchange. Fragging is a popular practice in the marine hobby.
G
GFO (Granular Ferric Oxide): A red/brown resin used in reactors or bags to absorb phosphates from saltwater or freshwater.
GH (General Hardness): Measures the total concentration of calcium and magnesium dissolved in water. Unit: °dGH.
Gonopodium: A modified anal fin in male viviparous fish (Poeciliidae) used to transfer sperm to the female.
GSA (Green Spot Algae): Green spots of algae on the glass. Difficult to remove; requires a stiff scraper. Thrives in conditions of low PO4 or high light.
H
Hardscape: Non-living elements of aquarium design: rocks, wood, substrate.
Hermaphrodite: An organism that possesses both sexes. Some shrimp, snails, and corals are sequential or simultaneous hermaphrodites.
Hexamita: A flagellated parasite that causes "hole-in-the-head disease" in cichlids. Treated with metronidazole.
HOB (Hang-On-Back): A filter that hangs from the outside of the aquarium over the back edge.
Holothurine: A toxin released by sea cucumbers (Holothuria) when they are stressed. It can contaminate the entire aquarium.
Hyposalinity: A technique for treating marine ich by reducing the salinity to 1.009–1.010 in the quarantine tank.
I
ICH (Ichthyophthirius multifiliis): A parasitic protozoan that causes white spot disease in freshwater fish. Very common. Treatable.
ICP-OES: Inductively Coupled Plasma Optical Emission Spectrometry. Measures more than 30 trace elements in seawater. Used by advanced aquarists.
Mouthbrooding: A reproductive strategy of certain African cichlids in which the mother (or both parents) incubates the eggs and fry in her mouth.
J / K
KH (carbonate hardness): Carbonate hardness. It measures bicarbonates and carbonates. The pH "buffer"—the higher the KH, the more stable the pH. Unit: °dKH.
Kalkwasser: A saturated solution of calcium hydroxide (Ca(OH)₂). It replenishes Ca and KH in saltwater aquariums and has an anti-algae effect.
Killifish: Fish of the families Nothobranchiidae and Cyprinodontidae. Many species are seasonal, and their eggs can survive in dry peat.
L
Labyrinth fish: Fish with a labyrinth organ that allows them to breathe atmospheric air. This group includes bettas and gouramis.
Larva: The juvenile stage of an aquatic animal, often with a different morphology than the adult.
Lateral line: A sensory organ on the sides of fish that detects vibrations, currents, and changes in pressure.
Live Rock: Natural reef rock colonized by microorganisms, small corals, and coralline algae. A natural biological filter for marine aquariums.
LPS (Large Polyp Stony): Large-polyp corals with a calcareous skeleton. They fall between soft corals and SPS in terms of difficulty.
M / N
Macroalgae: Larger, visible algae (Chaetomorpha, Caulerpa, Halimeda). They are cultivated in marine refuges to absorb NO3 and PO4.
Metamorphosis: A change in form during development. Marine fish have planktonic larvae that are very different from the adult form.
Molting (ecdysis): The process by which crustaceans shed their rigid exoskeleton in order to grow. The animal is left soft and vulnerable.
Nano aquarium: An aquarium smaller than 30 liters. Difficult to keep stable. For very small fish, shrimp, or specialized species.
Neocaridina: A genus of freshwater shrimp. N. davidi is the most popular species. They are hardy and easy to breed.
NSW (Natural Sea Water): Natural seawater. The NSW parameters serve as the benchmark for the coral reef.
Nitrobacter / Nitrospira: Bacteria that oxidize nitrites (NO2) to nitrates (NO3). The second stage of nitrification.
Nitrites (NO₂): A toxic nitrogen compound. An intermediate product of the nitrogen cycle. It blocks the transport of O₂ in fish blood.
Nitrates (NO3): The end product of the nitrogen cycle. Less toxic than NH3 and NO2. They accumulate over time → remove them by changing the water.
Nitrosomonas: Bacteria that oxidize ammonia (NH3) to nitrites (NO2). The first stage of nitrification.
O
Labyrinth organ: An accessory respiratory organ found in certain fish (bettas, gouramis) that allows them to absorb O2 from the air.
Osmosis: The movement of water through a semipermeable membrane from an area of lower solute concentration to one of higher solute concentration.
Reverse osmosis (RO): A process that forces water through a membrane under pressure, removing the vast majority of salts and contaminants.
Otocinclus (Oto): A small Loricariid fish (Otocinclus affinis and others) that is an excellent eater of green algae on leaves and glass.
P / Q
Paludarium: An aquarium that combines an aquatic zone with a terrestrial or emergent zone.
PAR (Photosynthetically Active Radiation): A measure of photosynthetically active radiation (μmol/m²/s). It indicates how much usable light is available for photosynthesis.
Pyelonephritis: A kidney infection in fish. It can cause hydrops (ascites, raised scales).
Pleopods: Abdominal appendages of crustaceans (shrimp). Females carry their eggs on them.
Polarization: The ability of SPS corals to be "polarized" in a specific direction of growth. Breaking this polarization causes stress.
Powerhead: Submersible circulation pump. Creates a current in the aquarium.
ppm (parts per million): A unit of concentration. In the aquarium hobby, mg/L ≈ ppm for dilute solutions.
PPT (parts per thousand): A unit of salinity. 35 ppt = 35 g of salt per kg of water = normal ocean salinity.
R
Refuge (Refugium): A compartment in the sump containing macroalgae, copepods, and/or benthic fauna. It filters the water naturally.
Refractometer: An instrument that measures the density of saltwater by measuring the refraction of light. Essential for marine use.
Rhizome: The horizontal stem of certain plants (Anubias, Microsorum). It should not be buried, or it will rot.
RODI: Water purified by reverse osmosis (RO) + deionizer (DI). TDS close to 0 ppm. Base water for Caridina shrimp and marine aquariums.
S
Yolk sac: A source of nutrients for newly hatched fry. They do not eat until it is depleted (2–4 days).
Salinity: The concentration of dissolved salts in water. The ocean has a salinity of ~35 ppt or a density of 1.025.
Schooling: Synchronized and coordinated schooling behavior. A defense against predators.
Shoaling: A social grouping of fish. Less coordinated than schooling.
Slime coat: A layer of mucus that protects the skin of fish. In discus, the fry feed on their parents' mucus.
SPS (Small Polyp Stony): Small-polyp corals with a calcareous skeleton. The most demanding corals in the marine hobby (Acropora, Montipora, Pocillopora).
Sump: An additional tank located beneath the main aquarium. It houses the equipment and increases the system's total volume.
Active substrate: A substrate that alters water parameters (acidifies, softens). Necessary for Caridina shrimp.
T
Buffer: A substance that resists changes in pH. KH is the primary buffer in freshwater aquariums.
Tannins: Organic compounds released by wood and leaves. They acidify the water and turn it brown. They occur naturally in the Amazonian biotope.
TDS (Total Dissolved Solids): The sum of all solids dissolved in water. Measured in ppm or μS/cm using a conductivity meter.
Thermocline: An area of abrupt temperature change between two layers of water. In deep aquariums, this can occur if the heater is located at the top.
Tomont: Ichthyophthirius cyst. It sinks to the bottom, and the parasites divide inside it. Resistant to treatment.
Trofonte: The stage of Ichthyophthirius that has settled on the fish's skin, where it causes visible symptoms.
U / V
Swim bladder: An organ in fish that aids in buoyancy. It regulates vertical position without expending energy.
Velvet (Oodinium): A dinoflagellate parasite. More dangerous than ich. Golden powder on the fish.
Viviparous: An organism that gives birth to fully formed live young (guppies, mollies).
W / X / Y / Z
Wabi-kusa: A substrate sphere with both emergent and submerged plants. An element of the Nature Aquarium.
Wabi-sabi: A Japanese aesthetic philosophy that finds beauty in imperfection and impermanence.
Xenia: A soft coral that "pulses" (pulsing xenia). It reproduces very quickly.
Zeolite: A porous mineral that captures ammonia. It is used to temporarily reduce NH3 levels. It is recharged with saltwater.
Zoanthus: A colonial soft coral. Very popular. The closely related Palythoa species contain palytoxin (toxic to humans).
Zooxanthellae (Symbiodinium): Symbiotic dinoflagellate algae that live within the tissues of corals, anemones, and some clams. They perform photosynthesis and provide up to 90% of the coral's energy.
Zoonosis: A disease that can be transmitted from animals to humans. Mycobacteriosis and Aeromonas are potential zoonoses in aquariums. Wash your hands after handling the aquarium.