Aquarium keeping has its own vocabulary, and most guides assume you already know it. This glossary defines the terms used across our cycling, fish, shrimp, planted tank and algae guides in plain language, and links each one to the guide that covers it in depth.
Water & the Nitrogen Cycle
Cycling
Growing the two groups of naturally occurring bacteria, mostly in the filter media, that convert fish waste into less harmful compounds. Most freshwater tanks finish in about 4 to 6 weeks. See how to cycle a tank.
Ammonia (NH3/NH4+)
The waste product fish release constantly through their gills and waste. The toxic un-ionized form (NH3) and the milder ionized form (NH4+) shift with pH and temperature, and home test kits report the total of both. See how to cycle a tank.
Nitrite (NO2-)
The intermediate product of the nitrogen cycle, nearly as unforgiving as ammonia. Any non-zero reading is a reason to act with a partial water change rather than to calculate how dangerous it is. See how to cycle a tank.
Nitrate (NO3-)
The end product of the nitrogen cycle, far less toxic than ammonia or nitrite and managed through routine water changes and live plants. See how to cycle a tank.
Fishless cycling
Dosing pure ammonia, or letting fish food or a shrimp decay, in an empty running tank while testing daily. It is the method most fish-health references recommend by default because no animal is exposed to toxic water. See fishless vs. fish-in cycling.
Fish-in cycling
Cycling with a small number of very hardy fish while managing ammonia and nitrite through daily testing and frequent partial water changes. It exposes the fish directly to spikes. See fishless vs. fish-in cycling.
GH (general hardness)
A measure of dissolved calcium and magnesium. Shrimp draw on it to rebuild their shells after every molt, and Neocaridina do best at roughly 6 to 12 dGH. See Neocaridina care.
KH (carbonate hardness)
A measure of the water's buffering against pH swings. Neocaridina target roughly 3 to 8 dKH, with 4 to 6 cited as ideal for pH stability. See Neocaridina care.
Parameter overlap
How well two species' temperature, pH and hardness ranges fit inside one tank. Being both "freshwater" is not enough, since a blackwater Amazon fish and a hard-water African rift lake fish would stress each other. See fish compatibility.
Fish & Stocking
Bioload
The amount of waste a tank's livestock produces for its filter and bacteria to process. Adding a full stocking list on day one overwhelms the bacterial colony. See beginner fish.
Adult size
The size a fish will reach, as opposed to the size it is sold at. A clown loach sold at 1 to 2 inches can pass a foot, so stocking should plan for the adult. See fish compatibility.
Group minimum
The smallest number of a schooling or social species that keeps it calm. Undersized groups redirect hierarchy behavior onto tankmates, as with tiger barbs kept in groups smaller than about eight. See beginner fish.
Fin-nipping
Biting at the long or flowing fins of slow tankmates. Often a symptom of an undersized group rather than a fixed trait of the species. See fish compatibility.
Quarantine
Holding new fish in a separate tank before adding them to an established one, so illness is caught early. A reliable store quarantines stock rather than shelving it straight off the truck. See beginner fish.
Shrimp
Neocaridina
The dwarf shrimp genus most people mean by "cherry shrimp" (Neocaridina davidi), sold in color lines like red cherry, yellow fire, blue dream and rili. More tolerant of everyday water conditions than Caridina. See Neocaridina care.
Caridina
The other main dwarf shrimp genus, including crystal red and black shrimp, Taiwan bees and tiger shrimp. Generally needs soft, more acidic water held in a narrow band. See Neocaridina care.
Molting
The shedding of a shrimp's exoskeleton as it grows. It draws on calcium and magnesium from the water, which is why general hardness matters. See Neocaridina care.
Plants & Aquascaping
Low-tech tank
A planted tank with no injected CO2, relying on dissolved CO2 plus what fish respiration and decomposition produce, kept with low-light, undemanding plants. The recommended starting point for a first planted tank. See your first planted tank.
High-tech tank
A planted tank that injects pressurized or DIY CO2 to push growth beyond what ambient CO2 supports. It adds equipment, cost and a demand for tighter balance. See your first planted tank.
Aquasoil
A pre-loaded, nutrient-rich planting substrate, sometimes called active substrate, that often buffers pH slightly downward. Forgiving for beginners, but costs more and can cloud the water for a few days. See your first planted tank.
Root tabs
Solid fertilizer capsules pushed into inert gravel or sand near root-feeding plants, since plain gravel or sand supplies no nutrients on its own. See your first planted tank.
PAR
Photosynthetically Active Radiation, a measure of how much light plants can actually use at a given point in the tank, as opposed to how bright a fixture looks to the eye. See your first planted tank.
Photoperiod
How many hours a day the tank light runs. A common low-risk start is 6 to 8 hours on a timer, since consistency prevents the swings that give algae an opening. See your first planted tank.
Algae
Diatoms (brown algae)
A soft brown dusty film that wipes off easily, typical of new tanks with silicates, light and an immature filter. See algae guide.
Green spot algae (GSA)
Small, hard green dots on glass and slow-growing leaves, usually from light combined with low CO2 or low phosphate. See algae guide.
Black beard algae (BBA)
Dark grey-black tufts on leaf edges, driftwood and high-flow areas, usually tied to unstable CO2 in a planted tank, often with excess organics. See algae guide.
Hair algae
Fine green filaments or threads that tangle around plants, often from a nutrient imbalance such as excess light or iron and nitrate relative to CO2. See algae guide.
Cyanobacteria (blue-green algae)
Not a true algae but a bacteria, forming a slimy blue-green or dark green sheet that peels off in one piece with a musty smell, usually from poor flow plus organic buildup. See algae guide.