🌿 Fishkeeping glossary

Aquarium terms, in plain language

Every hobby has its jargon — and fishkeeping is thick with it. Here are the words you'll meet in plain language, no gatekeeping and no assumed knowledge. 52 terms, each linked to the guide or tool that goes deeper. Because fishkeeping is water-keeping, we start with the cycle.

Updated September 2026

🔄 The nitrogen cycle

Nitrogen cyclethe cycle · biological cycleThe nitrogen cycle is the process where colonies of beneficial bacteria in your filter convert toxic fish waste (ammonia) first into nitrite, then into far less harmful nitrate — it is the single most important thing to understand in fishkeeping.Cyclingfishless cycle · fish-in cycleCycling is the act of establishing the beneficial bacteria colonies before (or while) stocking a tank, so the filter can neutralise ammonia and nitrite as fast as the fish produce them.AmmoniaNH3 · NH4Ammonia is the highly toxic nitrogen compound fish excrete and that rotting food and waste release; in a healthy cycled tank it should always read zero.NitriteNO2Nitrite is the second toxic stage of the nitrogen cycle — produced when bacteria break down ammonia — and like ammonia it should read zero in an established tank.NitrateNO3Nitrate is the far less toxic end-product of the nitrogen cycle, which builds up over time and is removed by regular partial water changes and (partly) by live plants.Beneficial bacterianitrifying bacteria · the biofilterBeneficial bacteria are the invisible nitrifying microbes that colonise your filter media and surfaces and do the actual work of the nitrogen cycle, turning ammonia into nitrite into nitrate.New tank syndromeNew tank syndrome is the wave of stressed, sick or dying fish caused by ammonia and nitrite spikes in a tank that was stocked before it finished cycling.

💧 Water & chemistry

pHpH measures how acidic or alkaline your water is on a scale of 0–14, with 7 neutral; most community fish are far more sensitive to sudden pH swings than to a stable value slightly off their ideal.GHgeneral hardness · total hardnessGH (general hardness) is the amount of dissolved minerals — mainly calcium and magnesium — in your water, and it matters more than pH for matching a fish to your tap water.KHcarbonate hardness · alkalinity · buffering capacityKH (carbonate hardness) is the water's buffering capacity — how well it resists changes in pH — so a healthy KH keeps your pH stable, while a near-zero KH lets pH crash.Water parametersparameters · water valuesWater parameters are the measurable qualities of your aquarium water — temperature, ammonia, nitrite, nitrate, pH, GH and KH — that together tell you whether the environment is safe.TDStotal dissolved solidsTDS (total dissolved solids) is a quick meter reading of everything dissolved in the water, used mainly by shrimp and soft-water breeders as a shortcut to track mineral content.Chlorine & chloraminetap water treatmentChlorine and chloramine are disinfectants added to tap water that are harmless to us but toxic to fish and to your beneficial bacteria, so all tap water must be treated with dechlorinator before it goes in the tank.Dechlorinatorwater conditionerDechlorinator (water conditioner) is a liquid additive that instantly neutralises chlorine and chloramine in tap water, making it safe for fish and filter bacteria.Water changepartial water change · PWCA water change is the routine removal of part of the tank water and its replacement with fresh, dechlorinated water — the main way you export nitrate and refresh minerals.Brackish waterBrackish water is water with a low, deliberate amount of salt — between fresh and marine — needed by a handful of specialist fish and by the larvae of some snails and shrimp, but not by ordinary community fish.

🏠 Setup & equipment

Filteraquarium filterA filter circulates the water and, most importantly, houses the beneficial bacteria that run the nitrogen cycle; its biological role matters far more than simply making the water look clear.Filter mediamedia · biomediaFilter media are the materials inside a filter — sponges, ceramic rings, floss and sometimes carbon — that trap debris and provide the huge surface area beneficial bacteria live on.Sponge filterA sponge filter is a simple air-driven filter that provides gentle flow and a large bacterial surface, making it the go-to choice for shrimp tanks, fry tanks and betta tanks where strong current is a problem.Heateraquarium heaterA heater keeps a tropical tank at a stable temperature; tropical fish need one, while coldwater fish such as goldfish and white cloud minnows do not.Substrategravel · sand · aquasoilSubstrate is the material on the tank floor — gravel, sand or nutrient-rich aquasoil — chosen to suit your fish and whether you plan to keep live plants.Gravel vacuumgravel siphon · siphonA gravel vacuum is a siphon tube used during water changes to draw out water while lifting trapped waste from the substrate.Cycled tankestablished tank · mature tankA cycled (established) tank is one whose filter bacteria can neutralise ammonia and nitrite to zero, and a mature tank is one that has run stably long enough to develop biofilm and stable chemistry.Nano tankA nano tank is a small aquarium (loosely, under about 40 litres) — appealing but less forgiving, because a small water volume lets temperature and chemistry swing faster.Community tankA community tank houses several peaceful species that share the same water and temperature needs and get along — the classic, rewarding beginner setup.

🐟 Behaviour

Shoaling & schoolingschooling fish · shoaling fishShoaling fish feel safe in numbers and schooling fish swim together in coordinated groups; both need to be kept in a proper group (commonly six or more) or they become stressed and sickly.Glass surfingpacing · glass dancingGlass surfing is when a fish repeatedly swims up and down the front glass; it is a stress signal, often from an uncycled tank, wrong parameters, boredom, or seeing its own reflection.Flashingglancing · gill flashing · scratchingFlashing is when a fish rubs or scrapes its body against surfaces; it signals irritation — often from poor water quality or an external parasite such as ich — and is a reason to check the water and observe closely.StressStress in fish is the underlying state — caused by bad water, wrong tankmates, too small a group, or an unsuitable tank — that weakens their immune system and lies behind most disease outbreaks.Labyrinth organlabyrinth fish · anabantoidThe labyrinth organ is a special breathing structure in bettas and gouramis that lets them gulp air from the surface, which is why you see them dart to the top — but it does not mean they can live in an unfiltered bowl.Normal vs abnormal behaviourLearning your fish's normal behaviour — how it swims, eats, rests and colours up — is the fastest way to spot a problem early, because a change in behaviour usually precedes visible illness.

🩺 Health

Ich (white spot)ick · white spot disease · IchthyophthiriusIch is a common parasite that shows as tiny white salt-like spots on a fish's body and fins, usually triggered by stress or a temperature drop; it is treatable if caught early.Fin rotFin rot is a bacterial condition where fins look ragged, torn or edged with white or black, almost always secondary to poor water quality or an injury — so improving the water is the first and biggest step.Swim bladder disorderSBDSwim bladder disorder is when a fish struggles to stay upright or control its depth — floating, sinking or swimming oddly — and in round-bodied fish like fancy goldfish and bettas it is often linked to diet and overfeeding.DropsyDropsy is not a disease but a symptom — a swollen belly with scales sticking out like a pinecone — that signals serious internal fluid buildup, often from organ failure, and carries a poor outlook.QuarantineQTQuarantine is keeping new fish (or sick fish) in a separate tank for a few weeks before adding them to your main tank, to avoid introducing disease to healthy stock.Hospital tanktreatment tankA hospital tank is a separate tank used to treat a sick fish away from the community, so medication does not harm other livestock, plants or filter bacteria.

🍽️ Feeding

OverfeedingOverfeeding is the most common beginner mistake — uneaten food rots and spikes ammonia while excess waste fuels algae and swim-bladder problems — so feeding small amounts is safer than generous ones.ConstipationConstipation in fish — often from too much dry food and not enough fibre — can cause bloating and swim-bladder trouble, and is usually eased by fasting and feeding foods like blanched, deshelled pea or daphnia.A varied dietfish food · dietA varied diet — a quality staple food plus occasional frozen or live foods and, for some fish, vegetable matter — keeps fish healthier and more colourful than a single dry food alone.

🌿 Plants & algae

AlgaeAlgae is the plant-like growth that appears on glass, décor and plants; some is normal and healthy, and an outbreak is usually a sign of too much light or too many nutrients rather than a disease.Photoperiodlighting periodThe photoperiod is how many hours a day your aquarium light is on; keeping it consistent and not too long (a timer helps) is one of the simplest ways to control algae.Planted tanklive plants · aquascapeA planted tank uses live aquarium plants, which improve water quality by taking up nitrogen, give fish shelter and compete with algae — and beginner-friendly species make it easier than it looks.BiofilmBiofilm is the thin, natural layer of micro-organisms that grows on surfaces in a maturing tank; it is a food source for shrimp, snails, fry and grazers like otocinclus, and a sign of a healthy establishing ecosystem.Hardscapedriftwood · rockHardscape is the non-living structure in a tank — driftwood, rocks and branches — used to create territory, hiding places and a natural layout; some woods also release tannins that tint the water.AquascapingAquascaping is the craft of arranging plants, hardscape and substrate into a deliberate underwater landscape — a rewarding creative side of the hobby that also benefits the fish when done with their needs in mind.Blackwatertannins · botanicalsA blackwater tank uses tannins from driftwood, leaf litter and seed pods to create soft, acidic, tea-stained water that mimics the natural habitat of many tetras, dwarf cichlids and bettas.

🥚 Breeding

Bioloadbiological loadBioload is the total amount of waste your livestock produces, which your filter and water changes must keep up with — a big messy fish like a goldfish has a far higher bioload than its size alone suggests.Stockingstocking level · overstockingStocking is choosing how many and which fish to keep in a tank so their adult size, bioload, group needs and temperaments all fit — the old "one inch per gallon" rule is unreliable and best ignored.LivebearerA livebearer is a fish that gives birth to free-swimming young rather than laying eggs — guppies, platies, mollies and swordtails — which means a mixed-sex group will breed readily, often faster than you expect.Frybaby fishFry are baby fish; raising them needs the right tiny foods, clean stable water and (usually) protection from adults, whether they arrive planned or as a surprise livebearer brood.BreedingBreeding is deliberately spawning fish and raising the young; it ranges from near-automatic (livebearers) to demanding (many egg-layers and dwarf cichlids), and responsible breeding means planning for where the offspring will go.