Glossary
Aquarium terms, plainly
The words that appear in every guide, explained without the jargon.
A
- Acclimation
- Gradually adjusting new fish to your tank’s temperature and water before release, to avoid the shock that causes stress and disease.Read the guide →
- Ammonia
- A toxic nitrogen waste produced by fish and decaying food. It is the first stage of the nitrogen cycle and should read zero in an established tank.Read the guide →
- Aquascape
- The art of arranging plants, wood and rock in an aquarium to create a natural underwater scene.
- Aufwuchs
- The layer of algae, biofilm and tiny organisms that coats surfaces in a mature tank. It is the natural food of grazers such as otocinclus and shrimp.
B
- Barbels
- The whisker-like sensory organs around the mouths of catfish and loaches. Rough gravel wears them down, so soft sand is kinder.
- Biofilm
- A thin film of bacteria and organic matter on surfaces, grazed by shrimp, snails and many small fish.
- Bioload
- The total amount of waste produced by all the animals in a tank. Larger fish add far more bioload than small ones.
- Blackwater
- Soft, acidic, tea-coloured water rich in tannins, mimicking Amazonian streams. Suited to species like cardinal tetras and chocolate gouramis.
- Brackish
- Water with a low, variable salinity — between fresh and marine — needed by a few species and by the larvae of nerite snails.
- Buffering
- The water’s ability to resist pH change. It comes mainly from carbonate hardness (KH); high buffering holds pH steady.
C
- Carbonate hardness (KH)
- A measure of the water’s buffering capacity, mostly from carbonates and bicarbonates. High KH holds pH steady; low KH lets pH drift.Read the guide →
- CO₂
- Carbon dioxide, added in planted tanks to boost plant growth. Excess threatens fish, so it is kept in a narrow range.Read the guide →
- Columnaris
- A fast-moving bacterial infection that shows as grey-white patches, often around the mouth. It is one of the most serious fish diseases.
- Community tank
- An aquarium housing several peaceful species that share similar water needs — chosen for compatibility, not just variety.Read the guide →
- Cycling
- Growing the colony of filter bacteria that convert toxic ammonia to nitrite and then to nitrate, before fish are added.Read the guide →
D
- Dechlorinator
- A water conditioner that neutralises the chlorine or chloramine in tap water, making it safe for fish and filter bacteria.Read the guide →
- dGH
- German degrees of general hardness. One degree equals about 17.85 ppm of dissolved calcium and magnesium.Read the guide →
- Dither fish
- Peaceful, active fish kept with a shy species to make it feel secure enough to come out and behave naturally.
- Drip acclimation
- A slow method of acclimating fish using airline tubing to drip tank water into their container, gentler than adding cupfuls.Read the guide →
- Dropsy
- Abnormal fluid accumulation, which may accompany swelling and raised scales. Appearance alone cannot identify the cause or predict recovery; seek prompt fish-health assessment.Read the guide →
F
- Filtration
- Cleaning tank water in three ways: mechanical (traps particles), biological (bacteria process waste) and chemical (removes odours or medicine).Read the guide →
- Fin rot
- A term for progressive fin deterioration that may involve infection. Damage alone cannot distinguish infection from injury; check ongoing tissue loss, water conditions and other signs, and seek fish-health assessment.Read the guide →
- Fishless cycling
- Cycling a new tank with an ammonia source but no fish, so the filter is ready and no fish suffers.Read the guide →
G
- General hardness (GH)
- The total dissolved calcium and magnesium in the water. Soft-water species need low GH; livebearers and African cichlids want it higher.Read the guide →
- Gravel vacuum
- A siphon that removes dirty water while lifting waste out of the substrate. Used during a water change, on rotation.Read the guide →
H
- Hard water
- Water with high levels of dissolved minerals (high GH and KH), typical of chalky areas. Suited to livebearers, snails and Rift Lake cichlids.Read the guide →
- Humane euthanasia
- Ending an animal’s life to prevent suffering when it cannot be relieved. For a fish, discuss welfare and an appropriate method with an aquatic veterinarian; a home remedy is not a guarantee of a painless death.
I
- Ich
- Freshwater white spot disease, caused by Ichthyophthirius multifiliis. White spots alone cannot confirm it; prompt professional assessment and a suitable treatment plan are needed. Raising the temperature is not a universal remedy.Read the guide →
L
- Labyrinth organ
- An air-breathing organ in bettas and gouramis that lets them take oxygen from the surface, so they cope with slow, low-oxygen water.
- Livebearer
- A fish that gives birth to free-swimming young rather than laying eggs, such as guppies, platies, mollies and swordtails.Read the guide →
M
- Mbuna
- The rock-dwelling cichlids of Lake Malawi — colourful, hardy and often aggressive, needing hard, alkaline water.
- Mouthbrooder
- A fish that carries its eggs or fry in its mouth for protection, common among African cichlids.
- Mulm
- The soft debris that collects in the substrate — uneaten food, waste and dead plant matter. It is removed with a gravel vacuum.
N
- Nitrate (NO₃)
- The end product of the nitrogen cycle. It is far less toxic than ammonia or nitrite but builds up and is removed by water changes.Read the guide →
- Nitrification
- The bacterial process that turns ammonia into nitrite and then into nitrate — the chemistry behind the nitrogen cycle.Read the guide →
- Nitrite (NO₂)
- The middle stage of the nitrogen cycle, toxic to fish. It should read zero once a tank is fully cycled.Read the guide →
- Nitrogen cycle
- The chain of bacteria that turns ammonia into nitrite and then into nitrate — the foundation of a healthy tank.Read the guide →
O
- Overflow
- A device that lets water leave the display tank and flow down to a sump, while keeping the water level constant.Read the guide →
- Overstocking
- Keeping more fish than the tank’s filtration and maintenance can support, which leads to rising ammonia and nitrate.
P
- pH
- A measure of how acidic or alkaline the water is, on a 0–14 scale. Stability matters more than an exact value for most fish.Read the guide →
- ppm
- Parts per million, a measure of concentration. In aquarium water it is equal to milligrams per litre (mg/L).
Q
- Quarantine
- Keeping new or unwell fish in a separate tank for a few weeks to watch for disease before they meet the others.Read the guide →
R
- Remineralise
- Adding minerals back to pure reverse-osmosis water so it suits fish or shrimp, setting GH and KH to a target.
- Reverse osmosis (RO) water
- Water with almost all minerals removed, used as a blank slate for soft-water tanks or shrimp — it must be remineralised or mixed with tap water.Read the guide →
- Rift Lake
- The hard, alkaline lakes of East Africa (Malawi, Tanganyika), home to the cichlids that need high-pH water.
S
- Shoaling
- The tendency of some fish to group together. Shoaling species feel safer in numbers and should be kept in groups.
- Substrate
- The material on the tank floor — gravel, sand or soil. Soft, rounded substrate protects the barbels of bottom-dwellers.Read the guide →
- Sump
- A second tank, usually below the main one, that holds filter media and equipment out of sight and adds water volume.Read the guide →
- Surface agitation
- Movement at the water surface, caused by filters or air stones, that drives oxygen into the water.
- Swim bladder
- A gas-filled organ involved in buoyancy control in many fish. Floating, sinking or rolling does not by itself confirm disease of this organ; environmental and other health problems also need assessment.Read the guide →
T
- Tannins
- Brown compounds released by driftwood and leaves that gently lower pH and tint the water like weak tea.
- TDS
- Total dissolved solids — a single number summarising everything dissolved in the water. Useful as a trend for shrimp and soft-water tanks.Read the guide →
V
- Velvet
- Freshwater velvet involves Piscinoodinium; marine velvet involves Amyloodinium. A golden sheen alone cannot diagnose either, and gill disease may occur without visible dust. Breathing distress needs urgent fish-health help.Read the guide →
W
- Water change
- Replacing part of the tank water with fresh, treated water to dilute nitrate and waste and replenish minerals.Read the guide →
Put it into practice
The Nitrogen Cycle and Cycling a TankThe nitrogen cycle is the foundation of every healthy tank. Understand it, and most “mystery” water problems stop being mysteries for good.How Often Should You Change Aquarium Water?Water changes remove waste that the filter cannot, and keep minerals topped up. There is no single number, but there is a simple way to find yours.How to Treat Ich (White Spot Disease)White spots need investigation, not a guessed remedy. Check urgent signs, understand the parasite’s life cycle, and prepare for treatment appropriate to the fish and aquarium.How Often Should You Feed Fish?Overfeeding causes more aquarium problems than underfeeding. Here is a realistic feeding routine, and the signs that you are giving too much.The Best Beginner Aquarium PlantsLive plants are not hard — some are almost unkillable. These easy, low-light species use up waste, look natural and give fish cover, with no special equipment.How to Choose an Aquarium FilterChoose an aquarium filter by media volume and flow, not just tank size. Here is how the main types compare and what really keeps the water safe.