The Aquarium Glossary

To gradually adjust a fish to new water conditions.

Acclimation

Slowly adjusting newly arrived fish to your tank's temperature and water before release.

Air Pump

A pump that pushes air through tubing to drive air stones and sponge filters.

Air Stone

A porous stone that diffuses pumped air into fine bubbles.

Algae

Photosynthetic growth that is natural in small amounts but unsightly in excess.

Algae Scraper

A tool used to clean algae off the aquarium glass.

Algae Wafer

A sinking disc of plant-based food for herbivores and bottom feeders.

Amano Shrimp

A larger algae-eating shrimp famous for its appetite for nuisance algae.

Ammonia

A highly toxic nitrogen compound released by fish waste, uneaten food, and decay.

Ammonia Spike

A sudden rise in ammonia, usually during cycling or after a disruption.

Aquarium Salt

Sodium chloride sometimes used as a mild treatment or tonic in freshwater tanks.

Aquascaping

The art of arranging plants, rocks, and wood to create a beautiful underwater landscape.

Aquasoil

A nutrient-rich, plant-focused substrate that also gently softens and acidifies water.

Assassin Snail

A snail that hunts and eats other snails, used to control pest snail outbreaks.

Auto Top-Off (ATO)

A system that automatically replaces water lost to evaporation.

B

Barbels

Whisker-like sensory organs around the mouth of fish such as catfish and loaches.

Beginner Fish

A hardy, forgiving species recommended for new aquarists.

Beneficial Bacteria

The invisible microbes that colonize your filter and surfaces and convert toxic waste into safer compounds.

Berried

A term for a female shrimp carrying eggs under her tail.

Biofilm

A thin layer of microorganisms that coats surfaces and serves as natural food.

Bioload

The total waste produced by everything living in the tank.

Biological Filtration

Filtration carried out by beneficial bacteria living on filter media.

Biotope

A tank that recreates the fish, plants, and decor of one natural habitat.

Black Beard Algae

Dark, tufted algae, often called BBA, that clings to leaves, wood, and equipment.

Blackwater

Soft, acidic, tannin-stained water that mimics tropical leaf-litter habitats.

Blanched Vegetables

Softened vegetables like zucchini, spinach, or cucumber offered to herbivores.

Bloodworms

The larvae of midge flies, a protein-rich treat fed frozen, live, or freeze-dried.

Bottom Dweller

A fish that spends most of its time on or near the tank floor.

Brackish

Water with a salinity between fresh and full marine, mimicking estuaries.

Brine Shrimp

Small crustaceans fed live, frozen, or hatched as newborns for fry.

Bubble Nest

A raft of saliva-coated bubbles built at the surface by male bettas and gouramis.

Buffer

A substance or product that stabilizes pH by resisting changes in acidity.

C

Camallanus Worms

Red, thread-like internal parasites that protrude from a fish's vent.

Canister Filter

A powerful sealed external filter, usually placed in the cabinet below the tank.

Captive-Bred

Fish raised in captivity rather than taken from the wild.

Caridina Shrimp

A group of shrimp, including Crystal and Bee shrimp, that need soft, acidic water.

Carnivore

A fish whose natural diet is primarily other animals.

Carpeting Plant

Low-growing plants that spread across the substrate like a lawn.

Caudal Fin

The tail fin, the main source of a fish's forward thrust.

Centerpiece Fish

A standout fish chosen as the visual focal point of a tank.

Check Valve

A one-way valve in airline tubing that stops water from back-siphoning into the air pump.

Chemical Filtration

Filtration that removes dissolved impurities, odors, and discoloration.

Cherry Shrimp

A hardy, colorful Neocaridina shrimp popular with beginners.

Chloramine

A more stable chlorine-and-ammonia disinfectant used by many water utilities.

Chlorine

A disinfectant added to municipal tap water that is toxic to fish and beneficial bacteria.

Cloudy Eye

A hazy or opaque film over one or both eyes.

CO2 (Carbon Dioxide)

A gas plants use for photosynthesis and a key lever in high-tech planted tanks.

CO2 Diffuser

A device that breaks pressurized CO2 into tiny bubbles for plants to absorb.

CO2 Injection

Adding pressurized carbon dioxide to dramatically boost plant growth.

Cold Water Tank

An unheated aquarium for fish that prefer cooler temperatures.

Columnaris

A fast-moving bacterial infection that produces white, cottony, or saddle-like patches.

Community Tank

A tank housing multiple peaceful species that coexist well.

Conditioning

Preparing fish to breed by feeding rich, varied foods and optimizing conditions.

Copper Sensitivity

The vulnerability of invertebrates to copper, which is toxic to them even in trace amounts.

Culling

Selectively removing offspring that do not meet breeding goals.

Cyanobacteria

A blue-green, slimy sheet, also called BGA, that is actually a bacteria, not true algae.

Cycling

Establishing colonies of beneficial bacteria in a new tank so it can safely process fish waste.

Cycling a Tank

The process of preparing a new aquarium's biological filter before adding fish.

D

Daphnia

Tiny water fleas fed as a treat that also aids digestion.

Dechlorinator

A water conditioner that neutralizes chlorine and chloramine in tap water.

Denitrification

The conversion of nitrate into nitrogen gas by anaerobic bacteria.

Detritus

Fine organic debris from waste, decaying plants, and leftover food that settles in the tank.

Diatoms

A brown, dusty algae common in newly set-up tanks.

Dissolved Oxygen

The amount of oxygen available in the water for fish and bacteria to breathe.

Dither Fish

Active, visible fish whose calm presence reassures shy or skittish tankmates.

DOA

Dead on Arrival, livestock that does not survive shipping.

Dorsal Fin

The fin running along a fish's back that aids stability and steering.

Dragon Stone

A textured, cratered reddish-brown rock popular in aquascaping.

Driftwood

Submerged wood used for decoration, structure, and shelter.

Drip Acclimation

A gentle acclimation method that slowly drips tank water into the container holding new arrivals.

Drop Checker

A small glass indicator that shows whether a planted tank's CO2 level is in a good range.

Dropsy

A condition where a fish swells and its scales pinecone outward due to fluid buildup.

Dry Start Method

Growing carpeting plants emersed in a humid, water-free tank before flooding it.

Dutch Style

An aquascaping style focused on dense, colorful arrangements of many plant species.

E

Egg Layer

A fish that reproduces by depositing eggs rather than bearing live young.

Epiphyte

A plant that grows attached to surfaces rather than rooted in substrate.

Euthanasia

The humane ending of a suffering fish's life when recovery is not possible.

Evaporation

The loss of water to the air, which concentrates minerals and salts left behind.

F

Fasting Day

A scheduled day with no feeding to aid digestion and prevent bloating.

Filter Media

The materials inside a filter that perform mechanical, biological, or chemical filtration.

Fin Nipping

Biting at the fins of tankmates, often from boredom, crowding, or wrong groupings.

Fin Rot

A bacterial infection that frays, discolors, and erodes the fins.

Fish-in Cycling

Cycling a tank that already has fish in it, requiring careful daily management.

Fishless Cycling

Cycling a tank with a bottled ammonia source instead of live fish.

Flake Food

Thin, dried flakes that are a staple diet for many community fish.

Floating Plant

Plants that drift on the surface with roots hanging into the water.

Flukes

Microscopic parasitic worms that infest the gills or skin.

Freshwater

An aquarium that uses fresh, non-salty water, by far the most common type.

Frozen Food

Frozen portions of foods like bloodworms, brine shrimp, and daphnia.

Fry

Newly hatched or very young fish.

Fungus

Fuzzy, cottony growth that appears on injured or weakened fish, eggs, or food.

G

Gallon

A common unit of aquarium volume, especially in the United States.

GH (General Hardness)

The concentration of calcium and magnesium dissolved in your water.

Gravel Vacuum

A siphon tool that removes debris from the substrate while draining water.

Gravid Spot

A dark patch near the vent of female livebearers that signals pregnancy.

Green Spot Algae

Hard green dots that form on glass and slow-growing leaves.

Gut Loading

Feeding nutritious food to live prey so it passes those nutrients to your fish.

H

Hardscape

The non-living structure of an aquascape: the rocks and wood.

Hardscape Scape

A layout built primarily from rock and wood with few or no plants.

Hardy

Describes a fish that tolerates a wide range of conditions and beginner mistakes.

Heater

A device that keeps water at a stable temperature suitable for tropical fish.

Herbivore

A fish that mainly eats plant and algae matter.

High-tech Tank

A planted tank using injected CO2, strong light, and regular fertilization.

HOB Filter

A hang-on-back filter that clips onto the tank rim and returns filtered water as a gentle waterfall.

Hole-in-the-Head

A condition, also linked to HLLE, that causes pitting and lesions on the head and lateral line.

Hospital Tank

A simple bare tank used to treat sick fish away from the display.

I

Ich

A common parasite, also called white spot disease, that appears as salt-like dots on fins and body.

Infusoria

Tiny aquatic microorganisms used as a first food for very small fry.

Internal Filter

A compact filter that sits fully submerged inside the tank.

Internal Parasite

Worms or protozoa living inside a fish's gut or organs.

Invertebrate

A tank inhabitant without a backbone, such as a shrimp, snail, or crab.

Iwagumi

A minimalist Japanese aquascaping style built around a careful arrangement of stones.

K

KH (Carbonate Hardness)

Your water's buffering capacity, or its ability to resist pH swings.

L

Labyrinth Organ

A specialized organ that lets certain fish breathe air directly from the surface.

Lateral Line

A sensory system along a fish's side that detects vibration and water movement.

LFS

Local Fish Shop, an independent aquarium store.

Light Timer

A device that turns aquarium lighting on and off on a fixed schedule.

Lighting

The light source over a tank, important for viewing, fish health, and plant growth.

Line Breeding

Selectively breeding related fish to fix and enhance desirable traits.

Liquid Fertilizer

Water-column nutrients dosed to support plant growth.

Live Food

Living prey such as brine shrimp, daphnia, or blackworms fed to fish.

Livebearer

A fish that gives birth to free-swimming young instead of laying eggs.

Low-tech Tank

A planted tank that runs without injected CO2 and with modest lighting.

M

Mature Tank

An established aquarium with a stable, well-developed bacteria colony.

Mechanical Filtration

The stage that physically traps debris and particles from the water.

Melt

The sudden dieback of plant leaves after they are introduced to a tank.

Mini-Cycle

A brief re-spike of ammonia or nitrite after the bacteria colony is disturbed.

Molting

The process where shrimp and other crustaceans shed their exoskeleton to grow.

Mouthbrooder

A fish that incubates eggs and fry inside its mouth for protection.

MTS

Multiple Tank Syndrome, the affectionate term for a hobbyist's growing collection of tanks.

Mulm

The soft, dark organic sludge that collects in filters and substrate.

Mystery Snail

A large, peaceful snail available in colors like gold, blue, and ivory.

N

Nano Tank

A small aquarium, generally under about ten gallons.

Nature Aquarium

A style pioneered by Takashi Amano that recreates natural landscapes underwater.

Nerite Snail

A popular algae-eating snail that will not overpopulate freshwater tanks.

New Tank Syndrome

Fish illness or death caused by adding them to an un-cycled tank.

Nitrate

The relatively harmless end product of the nitrogen cycle that accumulates over time.

Nitrifying Bacteria

The specific bacteria that perform the nitrogen cycle's conversions.

Nitrite

A toxic compound produced as bacteria break down ammonia, forming the second stage of the nitrogen cycle.

Nitrogen Cycle

The natural process where bacteria convert toxic ammonia into nitrite, then into far less harmful nitrate.

Nocturnal

Active mainly at night and resting during the day.

O

Old Tank Syndrome

A slow decline in water quality from neglected maintenance over months or years.

Omnivore

A fish that eats both plant and animal foods.

Overfeeding

Giving more food than fish can eat, polluting the water.

Overstocking

Keeping more fish than the tank and filtration can healthily handle.

P

Partial Water Change

Swapping out only a fraction of the tank's water at a time.

Pearling

Tiny oxygen bubbles forming on plant leaves during active photosynthesis.

Pellet

Compressed dry food made in sinking or floating forms for different feeders.

Pest Snail

Small snails like bladder and ramshorn that arrive uninvited and multiply quickly.

pH

A 0-14 scale measuring how acidic or alkaline your water is.

pH Crash

A sudden, dangerous drop in pH usually caused by depleted KH buffering.

Phosphate

A nutrient (PO4) from food and waste that, in excess, can drive algae growth.

Photoperiod

The number of hours the aquarium light is on each day.

Planted Tank

An aquarium designed around living aquatic plants.

Pop-eye

Swelling that causes one or both eyes to bulge from the socket.

Powerhead

A small submersible pump that adds water movement and circulation.

Protein Skimmer

A device that removes dissolved organic waste from saltwater before it breaks down.

Q

Quarantine (QT)

Isolating new or sick fish in a separate tank before they join the main aquarium.

Quarantine Tank

A dedicated tank for isolating and observing new or sick fish.

R

Reef Tank

A marine aquarium centered on keeping live corals alongside fish.

Remineralization

Adding controlled minerals back to pure RO or rainwater to reach target GH and KH.

Rhizome Plant

Plants like Anubias and Java fern that grow from a horizontal rhizome.

RO Water (Reverse Osmosis)

Highly purified water stripped of minerals and contaminants by an RO membrane.

Root Tabs

Fertilizer capsules pushed into the substrate to feed root-hungry plants.

S

Salinity

The concentration of dissolved salts in water, central to brackish and marine tanks.

Saltwater

A marine aquarium that recreates ocean conditions for saltwater fish and invertebrates.

Schooling Fish

Species that swim together in coordinated groups for safety and comfort.

Seeding

Jump-starting a new tank using established media or substrate from a mature one.

Seiryu Stone

A blue-grey rock with white veining favored in Iwagumi layouts.

Sexing

Determining whether a fish is male or female.

Sexual Dimorphism

Visible physical differences between males and females of a species.

Shoaling

Loosely grouping with others of the same species without strict coordinated movement.

Slime Coat

The protective mucus layer covering a fish's body.

Spawn

The act of fish releasing and fertilizing eggs, or the eggs themselves.

Species-Only Tank

A tank dedicated to a single species.

Specific Gravity

A measure of water density used to track salinity in saltwater and brackish tanks.

Spider Wood

A branchy, twisting type of driftwood prized for dramatic aquascapes.

Sponge Filter

A simple air-driven filter that pulls water through a porous sponge.

Stem Plant

Fast-growing plants with a central stem that are trimmed and replanted to propagate.

Stocking

How many and which species a tank can comfortably support.

Strain

A distinct, selectively bred variety within a species.

Stress

A physiological strain from poor conditions, aggression, or sudden change that weakens fish.

Substrate

The material covering the bottom of the tank, such as gravel, sand, or aquasoil.

Sump

A separate tank, usually below the display, that houses filtration and equipment.

Surface Agitation

Movement at the water's surface that drives oxygen exchange.

Swim Bladder Disorder

A buoyancy problem that leaves a fish floating, sinking, or swimming sideways.

T

Tank Footprint

The length-by-width floor area of an aquarium.

Tannins

Natural compounds released by driftwood, leaves, and botanicals that tint water amber.

TDS

Total Dissolved Solids, a quick measure of everything dissolved in your water.

Territorial

Defending a chosen area of the tank against intruders, especially during breeding.

Test Kit

A set of reagents or strips used to measure water parameters like ammonia, nitrite, nitrate, and pH.

Thermometer

A tool that displays the actual water temperature in the tank.

Top-Off

Adding fresh water to replace what has evaporated.

U

Understocking

Keeping fewer fish than a tank could support.

UV Sterilizer

A unit that passes water past ultraviolet light to kill free-floating algae, bacteria, and parasites.

V

Velvet