Aquarium Fish Glossary

Acclimation: The process of slowly introducing fish to a new aquarium environment to prevent shock from sudden changes in water parameters. This is typically done by floating the bag containing the fish in the aquarium to equalize temperature, then gradually adding small amounts of tank water to the bag over a period of time.

Activated Carbon: A porous form of carbon used in aquarium filters to remove organic pollutants, chemicals, and odors from water. Activated carbon works through adsorption, where dissolved particles stick to the surface of the carbon. It is effective at making water clearer and removing impurities but needs to be replaced regularly.

Aeration: The introduction of air into aquarium water to increase oxygen saturation and promote gas exchange. Aeration is typically achieved using devices such as air pumps, airstones, or diffusers. Proper aeration is crucial for the health of fish and beneficial bacteria, especially in densely stocked tanks.

Algae: Simple photosynthetic organisms that can grow in aquariums, often seen as green, brown, or red growths. While some algae can be beneficial, providing oxygen and serving as food for some fish, excessive algae growth can indicate imbalances in nutrients or lighting and can detract from the aesthetic of the tank.

Ammonia (NH3): A toxic waste product produced by fish and from the decomposition of organic matter such as uneaten food and plant debris. In a cycled aquarium, beneficial bacteria convert ammonia into nitrite and then nitrate, which are less harmful. High ammonia levels can lead to ammonia poisoning, which is often fatal to fish.

Aquascaping: The art and practice of arranging aquatic plants, rocks, driftwood, and substrate in an aesthetically pleasing manner within an aquarium. Aquascaping is not only about creating a visually appealing environment but also about providing a functional habitat for fish and other aquatic life.

Bioload: The total amount of living biological matter in an aquarium, including fish, plants, and microorganisms, which contributes to the production of waste. A higher bioload increases the demand on the filtration system and can lead to water quality issues if not managed properly.

Brackish Water: Water with a salinity level between freshwater and seawater, typically between 1.002 and 1.022 specific gravity. Brackish water environments are home to certain fish species, such as pufferfish and gobies, and require careful monitoring of salinity levels.

Carnivorous Fish: Fish that primarily feed on animal-based foods, such as insects, smaller fish, or prepared meaty foods. Carnivorous fish require a diet high in protein, and their feeding habits should be considered when selecting tank mates to prevent predation.

Cycling: The process of establishing a biological filtration system in a new aquarium by growing colonies of beneficial bacteria that convert ammonia to nitrite and then to nitrate. Cycling is a critical step in setting up an aquarium to ensure a safe and stable environment for fish.

Diatoms: A type of brown algae that commonly appears in new aquariums as a thin, brown, dusty coating on surfaces. Diatoms can be a sign of excess silicates or nutrients in the water and usually diminish as the tank matures.

Disease Quarantine: The practice of isolating new or sick fish in a separate tank to prevent the spread of disease to the main aquarium. Quarantine tanks allow for observation and treatment of fish without risking the health of the entire aquarium population.

Ecosystem: In the context of aquariums, an ecosystem refers to the complex interrelationships between the fish, plants, microorganisms, and the physical environment within the tank. A balanced aquarium ecosystem is key to sustaining healthy aquatic life.

Filtration: The process of removing physical, chemical, and biological contaminants from aquarium water to maintain a clean and healthy environment. Filtration systems typically include mechanical, chemical, and biological components, each serving a different purpose in water purification.

Fry: The term for the young or offspring of fish, which are often much smaller and more vulnerable than adult fish. Fry may require special care, such as a separate breeding tank and finely ground or specialized food to ensure proper growth and development.

General Hardness (GH): A measure of the concentration of magnesium and calcium ions in water, which affects the water’s hardness. Some fish species have specific GH requirements, and maintaining appropriate levels is important for their health and well-being.

Herbivorous Fish: Fish that primarily feed on plant material, including algae and vegetables. These fish require diets rich in fiber and may benefit from supplements such as spirulina or blanched vegetables to meet their nutritional needs.

Ichthyophthirius multifiliis (Ich): A common and highly contagious parasitic disease in freshwater fish, also known as “white spot disease.” It is characterized by small, white cysts on the skin, fins, and gills, and requires prompt treatment to prevent widespread infection and mortality.

Invertebrates: Animals without a backbone, such as snails, shrimp, and corals, that are often kept in aquariums. Invertebrates can play important roles in the tank ecosystem, from algae control to being part of the clean-up crew.

KH (Carbonate Hardness): A measure of the concentration of bicarbonate and carbonate ions in water, which determines the water’s buffering capacity and ability to resist pH changes. Maintaining stable KH levels is important for preventing sudden pH swings that can stress or harm aquatic life.

Live Rock: Porous rocks from oceanic environments that are often used in marine aquariums to provide biological filtration and habitat for marine organisms. Live rock is populated with beneficial bacteria, algae, and small invertebrates that contribute to the natural balance of a reef tank.

Macroalgae: Large, multicellular algae that are often used in marine aquariums to absorb excess nutrients, provide habitat, and enhance the aesthetic of the tank. Some macroalgae species can also help to outcompete nuisance algae for resources.

Nitrates (NO3): The final byproduct of the nitrogen cycle, nitrates are less toxic than ammonia or nitrites but can still harm fish and promote algae growth at high levels. Regular water changes and live plants can help manage nitrate levels in an aquarium.

Nitrites (NO2): A toxic compound produced by beneficial bacteria as they break down ammonia in the nitrogen cycle. Nitrites should be kept at near-zero levels in a well-cycled aquarium to ensure the safety and health of the fish.

Omnivorous Fish: Fish that consume both plant and animal matter as part of their diet. Omnivores require a balanced diet that includes a variety of foods to meet their nutritional needs.

Osmosis: The diffusion of water across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. In fishkeeping, osmosis affects how fish regulate water and salt within their bodies, which is especially important in saltwater and brackish setups.

Overfeeding: The practice of offering more food to aquarium inhabitants than they can consume within a reasonable amount of time, typically a few minutes. Overfeeding can lead to poor water quality by increasing nutrient levels and contributing to the buildup of waste products, which can result in algae growth and harm the health of the fish.

pH: A scale that measures the acidity or alkalinity of water, with 7.0 being neutral, values below 7.0 acidic, and above 7.0 alkaline. Different species of fish require different pH levels to thrive, and maintaining a stable pH is crucial for the well-being of aquarium inhabitants.

Photosynthesis: The process by which aquatic plants and some algae use light to convert carbon dioxide and water into nutrients (sugars) and oxygen. Photosynthesis is essential for plant growth and contributes to the oxygenation of the water, benefiting all tank inhabitants.

Plankton: Microscopic organisms that float or drift in water, which can include both phytoplankton (plant-like) and zooplankton (animal-like). Plankton is a key food source for many filter-feeding aquatic organisms and plays a vital role in the aquatic food chain.

Protein Skimmer: A device used primarily in saltwater aquariums to remove organic waste compounds from the water by creating a column of microbubbles that proteins and other waste attach to and are then removed as foam. Protein skimmers help maintain water quality and reduce the load on other filtration systems.

Quarantine Tank: A separate aquarium used to isolate new, sick, or aggressive fish to prevent the spread of disease, parasites, or conflict within the main display tank. Quarantine tanks are an essential tool for responsible fishkeeping and help maintain the health of the aquarium community.

Reef Safe: A term used to describe fish, invertebrates, and products that are considered safe for use in a reef aquarium, meaning they will not harm corals or other sensitive reef inhabitants. It is important to research and select reef-safe options to ensure the longevity and stability of a reef ecosystem.

Salinity: The concentration of dissolved salts in water, measured by specific gravity or parts per thousand (ppt). Salinity is a critical parameter in marine and brackish water aquariums, and maintaining the appropriate levels is vital for the health of the organisms living in those environments.

Substrate: The material that lines the bottom of an aquarium, which can range from gravel and sand to specialized planted tank soils. Substrate can influence water chemistry, provide a surface for beneficial bacteria to colonize, and contribute to the overall aesthetics of the tank.

Sump: An additional tank or container that is connected to the main aquarium, typically located below it, which houses filtration equipment, provides additional water volume, and can host refugiums or other specialized systems. Sumps are especially popular in larger freshwater and marine aquariums.

Supplementation: The addition of essential nutrients, minerals, or vitamins to the aquarium to support the health and growth of fish, plants, and invertebrates. Supplementation is particularly important in reef tanks, where corals and other invertebrates may require specific elements to thrive.

Surface Agitation: The disturbance of the water’s surface, which increases oxygen exchange and helps prevent the formation of a surface film. Surface agitation can be achieved through the use of air stones, powerheads, water pumps, or by the filter output breaking the surface tension.

Total Dissolved Solids (TDS): A measure of all the organic and inorganic substances dissolved in the water, including minerals, salts, metals, and cations or anions dissolved in the water. TDS can affect the health of aquarium inhabitants and is often monitored in advanced fishkeeping and breeding practices.

Trace Elements: Essential minerals required in very small amounts for the biological processes of aquatic plants and animals. In reef aquariums, trace elements such as iodine, iron, and strontium must be replenished regularly to support the growth and health of corals and invertebrates.

UV Sterilizer: A device that uses ultraviolet light to kill or neutralize harmful microorganisms, such as bacteria, viruses, and algae spores, in aquarium water. UV sterilizers are used as a tool to improve water clarity and prevent the spread of disease in both freshwater and marine aquariums.

Water Change: The routine practice of removing a portion of the aquarium’s water and replacing it with fresh, treated water to dilute accumulated waste products and replenish essential minerals. Water changes are fundamental to maintaining water quality and the health of the aquarium ecosystem.

Water Conditioning: The process of treating tap water with chemicals or filtration systems to remove chlorine, chloramines, heavy metals, and other impurities before adding it to an aquarium. Proper water conditioning is essential to prevent harm to fish and other aquatic life.

Wet/Dry Filter: A type of biological filtration system that exposes beneficial bacteria to both air and water to improve the efficiency of the nitrogen cycle. Wet/dry filters are often used in larger aquariums and are known for their high biological filtration capacity.

Zooplankton: Small, often microscopic animals that drift in the water column, serving as an essential food source for many aquatic species, particularly filter feeders and larval fish. Zooplankton can include crustaceans, rotifers, and the larval stages of larger animals.

Biotope: An aquarium setup designed to replicate a specific natural habitat, including the fish, plants, substrate, and water conditions found in that environment. Biotopes provide a more authentic experience and can be educational in understanding the ecological dynamics of a particular region.

Detritus: Organic waste material that accumulates on the bottom of an aquarium, consisting of decomposing plant matter, uneaten food, fish waste, and other debris. Regular cleaning and proper filtration help to manage detritus and maintain water quality.

Live Food: Food for aquarium fish that is still alive, such as brine shrimp, daphnia, or bloodworms. Live food can provide enrichment and essential nutrients, but must be sourced from reputable suppliers to avoid introducing diseases.

Refugium: A separate compartment or tank connected to the main aquarium that provides a safe haven for microfauna, such as copepods and amphipods, and can also be used to grow macroalgae that help control nutrients. Refugiums are particularly beneficial in reef aquariums.

RO/DI Water: Water that has been purified through reverse osmosis (RO) and deionization (DI) processes to remove virtually all impurities. RO/DI water is often used in marine and freshwater aquariums to prevent unwanted chemicals and minerals from affecting water quality and fish health.

Alkalinity: A measure of the water’s capacity to neutralize acids and maintain a stable pH, often associated with the bicarbonate and carbonate content of the water. Alkalinity is an important parameter in both freshwater and marine aquariums, as it can impact the health of fish and invertebrates.

Canister Filter: A type of external aquarium filter that is particularly effective for mechanical, chemical, and biological filtration. Canister filters are popular due to their large media capacity, versatility, and the ability to handle larger aquariums with high bioloads.

Caudal Fin: The tail fin of a fish, which is used for propulsion. The shape and size of the caudal fin can vary greatly among fish species and can be an indicator of the fish’s swimming ability and behavior.

Clamped Fins: A condition where a fish holds its fins close to the body rather than spreading them out, often a sign of stress, poor water quality, or disease. Observing a fish’s fins can provide early indications of potential health issues.

Copepods: Small crustaceans found in marine and freshwater habitats that are a key component of the aquatic food web, serving as a food source for many fish species, particularly those that are filter feeders.

Coral Bleaching: A phenomenon where corals lose their vibrant colors and turn white due to stress, often caused by changes in temperature, light, or water quality. Coral bleaching can lead to the death of the coral if the stressors are not mitigated.

Corydoras: A genus of freshwater catfish known for their peaceful nature and distinctive armored plates. Corydoras are popular in community aquariums and are valued for their bottom-feeding behavior, which helps keep the substrate clean.

Cryptocaryon irritans: The marine counterpart to freshwater Ich, commonly known as “marine ich” or “saltwater ich,” which causes white spots on the skin and gills of infected fish and requires different treatment methods than freshwater Ich.

Cyanobacteria: Also known as “blue-green algae,” these bacteria can form slimy mats in aquariums and are often indicative of poor water quality or excess nutrients. Management of cyanobacteria includes improving water flow, reducing light, and maintaining proper nutrient levels.

Diurnal: Refers to fish or other aquatic organisms that are active during the day and rest at night. Understanding the diurnal or nocturnal nature of fish can help aquarists provide appropriate lighting and feeding schedules.

Dorsal Fin: The fin located on the back of a fish, which helps to stabilize the fish during swimming and can vary in size and shape across different species. The dorsal fin can also be a distinctive feature used in identifying fish species.

Egg Scatterer: A type of fish that reproduces by scattering eggs over substrate or plants rather than exhibiting parental care. Many cyprinids and characins are egg scatterers, and breeding them in captivity may require specific conditions to protect the eggs from being eaten.

Fallow Period: A period of time during which a reef aquarium is left without any fish to break the life cycle of parasites like marine ich. A fallow period can be an effective method for dealing with persistent parasitic diseases.

Filtration Media: The various materials used in aquarium filters to trap debris (mechanical filtration), remove dissolved wastes (chemical filtration), or provide a surface for beneficial bacteria to grow (biological filtration). Examples include sponge, activated carbon, ceramic rings, and bio-balls.

Gas Exchange: The process by which oxygen enters the aquarium water and carbon dioxide is expelled, which is crucial for maintaining a healthy environment for aquatic life. Gas exchange is facilitated by surface agitation and proper aeration.

Gravid: A term used to describe a female fish that is full of eggs and ready to spawn. Gravid females may exhibit changes in behavior and appearance, such as a more rounded belly and intensified coloration.

Hydrometer: An instrument used to measure the specific gravity or salinity of aquarium water, which is particularly important in marine and brackish water setups to ensure the proper environment for the inhabitants.

Lateral Line: A sensory organ found in fish that runs along the sides of the body and is used to detect vibrations and pressure changes in the water. The lateral line helps fish navigate, avoid predators, and locate prey.

Live Bearer: A type of fish that gives birth to live young instead of laying eggs. Common live-bearing fish in the aquarium trade include guppies, mollies, platies, and swordtails.

Marine Velvet: A parasitic disease caused by dinoflagellates, such as Amyloodinium ocellatum, that results in a velvety dusting on the skin of marine fish. Marine velvet is highly contagious and can be lethal if not treated promptly.

Mysis Shrimp: Small crustaceans often used as a high-quality food source for both freshwater and marine aquarium fish. Mysis shrimp are rich in protein and fatty acids, making them an excellent dietary supplement.

Nano Tank: A small aquarium, typically less than 30 gallons, that can house a variety of small fish, invertebrates, and corals. Nano tanks require careful maintenance due to their limited water volume, which can lead to rapid changes in water parameters.

Nitrogen Cycle: The biological process by which beneficial bacteria convert toxic ammonia into nitrites and then into less harmful nitrates. The nitrogen cycle is fundamental to establishing a healthy and stable aquarium.

Oscillation: In the context of aquariums, oscillation refers to the back-and-forth movement of water, often created by wave-making devices. Oscillation can improve water circulation, gas exchange, and provide a more natural environment for certain fish and corals.

Pelvic Fins: A pair of fins located on the underside of a fish, near the head, which are used for steering and maintaining stability during swimming. In some species, the pelvic fins can also be used to sift through the substrate or assist in nesting behaviors.

Biological Filtration: The process in which beneficial bacteria break down ammonia and nitrite into less harmful nitrate. This natural filtration method is essential for maintaining a healthy aquarium environment and requires proper media and conditions to support the bacteria.

Chloramine: A chemical compound of chlorine and ammonia used in municipal water supplies to disinfect water. Chloramine must be neutralized before being added to an aquarium, as it is toxic to fish and beneficial bacteria.

Dissolved Oxygen (DO): The amount of oxygen that is dissolved in the aquarium water, which is vital for the respiration of fish and other aquatic organisms. Adequate dissolved oxygen levels are maintained through aeration and proper circulation.

External Parasites: Organisms that live on the skin, gills, or scales of fish, feeding on their blood or tissue. Common external parasites in aquariums include Ichthyophthirius multifiliis (Ich), flukes, and lice, and they often require targeted treatments to eradicate.

Fungus: A type of organism that can appear in aquariums as cotton-like growths, often affecting fish with compromised immune systems or open wounds. Fungal infections require prompt treatment with antifungal medications to prevent spreading and to protect the health of the fish.

Gill Flukes: Parasitic flatworms that infect the gills of fish, causing irritation, increased mucus production, and difficulty breathing. Treatment for gill flukes typically involves the use of antiparasitic medications and maintaining good water quality.

Hard Water: Water with a high mineral content, particularly calcium and magnesium, which results in a higher general hardness (GH). Some fish species thrive in hard water, while others require soft water to flourish.

In situ: A Latin term meaning “in the original place,” used in aquarium parlance to describe conditions or behaviors observed in an organism’s natural habitat, as opposed to in captivity.

Jaw Locking: A behavior observed in some cichlid species where two individuals lock jaws as a form of aggression or during courtship rituals. Jaw locking is a natural behavior and can be a sign of territorial disputes or breeding activity within the aquarium.

Kalkwasser: A German term for limewater, a solution of calcium hydroxide used in marine aquariums to supplement calcium and maintain alkalinity, which is essential for coral growth.

LED Lighting: Light-emitting diode (LED) technology used for aquarium lighting that is energy-efficient, produces less heat, and can be programmed to simulate natural lighting conditions, including sunrise, sunset, and moonlight phases.

Microfauna: Small, often microscopic, animals that live in the aquarium substrate or water column, including copepods, amphipods, and rotifers. Microfauna contribute to the tank’s biodiversity and can serve as a food source for fish and corals.

Nitrogenous Waste: Waste products in the aquarium that contain nitrogen, such as ammonia, nitrite, and nitrate, which are produced by fish respiration, excretion, and the breakdown of organic matter.

Otoliths: Small, calcified structures found in the inner ear of fish that aid in balance and hearing. Otoliths can also be used by scientists to determine the age of a fish by counting growth rings, similar to tree rings.

pH Shock: A sudden change in the pH level of aquarium water that can cause stress or even death to fish. pH shock is often the result of improper water changes or the addition of substances that alter the water chemistry too quickly.

Quarantine Protocol: The recommended procedure for isolating new or sick fish in a separate tank for a period of time to monitor for signs of disease or parasites before introducing them to the main aquarium.

Reverse Osmosis (RO): A water purification process that uses a semipermeable membrane to remove impurities, including dissolved salts, bacteria, and chemicals, from water. RO systems are commonly used in aquarium keeping to provide pure water as a base for creating specific water conditions.

Soft Water: Water with a low mineral content, resulting in a lower general hardness (GH). Soft water is preferred by certain fish species, such as those from the Amazon River Basin, and may require the use of water conditioners or RO water to achieve the desired hardness.

Tannins: Organic compounds released into the water from natural decor items like driftwood and leaves, which can lower the pH and give the water a tea-like color. Tannins are often embraced in biotope aquariums to simulate natural blackwater conditions.

Undergravel Filter: A type of biological filtration system that uses a perforated plate under the substrate to draw water down through the gravel, where beneficial bacteria break down waste products. Undergravel filters are less common in modern aquarium keeping but can be effective in specific setups.

Venturi: A device or feature in some aquarium equipment that creates a suction effect, drawing air into the water stream to increase oxygenation. Venturi systems are often used in protein skimmers and powerheads.

Water Hardness: The measure of dissolved calcium and magnesium ions in the water, which contributes to the general hardness (GH) and carbonate hardness (KH) of the water. Water hardness affects the health and breeding of aquarium fish and plants.

Xenotoca: A genus of livebearing fish native to Central America, known for their bright colors and ease of breeding in captivity. Xenotoca species, such as Xenotoca eiseni, are popular among hobbyists who enjoy livebearers.

Yolk Sac: The nutrient-rich sac that feeds a developing fish embryo before hatching and can continue to provide nourishment during the early larval stage. In egg-laying fish, the yolk sac is visible through the transparent egg casing.

Zeolite: A microporous, aluminosilicate mineral used in aquarium filtration to adsorb ammonia and other nitrogenous compounds. Zeolite is particularly useful during the initial setup of an aquarium or in emergency situations where ammonia levels need to be quickly reduced.

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