🌿 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 cycle the cycle · biological cycle The 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. Cycling fishless cycle · fish-in cycle Cycling 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. Ammonia NH3 · NH4 Ammonia 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. Nitrite NO2 Nitrite 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. Nitrate NO3 Nitrate 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 bacteria nitrifying bacteria · the biofilter Beneficial 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 syndrome New 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

pH pH 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. GH general hardness · total hardness GH (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. KH carbonate hardness · alkalinity · buffering capacity KH (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 parameters parameters · water values Water 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. TDS total dissolved solids TDS (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 & chloramine tap water treatment Chlorine 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. Dechlorinator water conditioner Dechlorinator (water conditioner) is a liquid additive that instantly neutralises chlorine and chloramine in tap water, making it safe for fish and filter bacteria. Water change partial water change · PWC A 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 water Brackish 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

Filter aquarium filter A 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 media media · biomedia Filter 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 filter A 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. Heater aquarium heater A 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. Substrate gravel · sand · aquasoil Substrate 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 vacuum gravel siphon · siphon A gravel vacuum is a siphon tube used during water changes to draw out water while lifting trapped waste from the substrate. Cycled tank established tank · mature tank A 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 tank A 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 tank A community tank houses several peaceful species that share the same water and temperature needs and get along — the classic, rewarding beginner setup.

🐟 Behaviour

Shoaling & schooling schooling fish · shoaling fish Shoaling 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 surfing pacing · glass dancing Glass 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. Flashing glancing · gill flashing · scratching Flashing 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. Stress Stress 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 organ labyrinth fish · anabantoid The 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 behaviour Learning 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 · Ichthyophthirius Ich 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 rot Fin 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 disorder SBD Swim 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. Dropsy Dropsy 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. Quarantine QT Quarantine 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 tank treatment tank A 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

Overfeeding Overfeeding 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. Constipation Constipation 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 diet fish food · diet A 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

Algae Algae 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. Photoperiod lighting period The 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 tank live plants · aquascape A 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. Biofilm Biofilm 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. Hardscape driftwood · rock Hardscape 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. Aquascaping Aquascaping 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. Blackwater tannins · botanicals A 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

Bioload biological load Bioload 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. Stocking stocking level · overstocking Stocking 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. Livebearer A 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. Fry baby fish Fry 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. Breeding Breeding 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.