Reference

Aquarium Water Glossary

Every term here is one you will meet on a product label, in a test kit instruction sheet, or in a forum thread that assumes you already know it. The definitions are short on purpose. Where a term matters in practice, the links take you to the page where it does the work.

26 terms · 26 linked to a page that uses them

Activated carbon: A porous form of carbon used as chemical filtration. It adsorbs dissolved organic compounds, tannins and many odors, which is why it clears yellow water quickly. It does not remove ammonia, nitrite or nitrate in any useful quantity, and it stops working after roughly a month.Where it mattersFilter Media & Cartridges · The water is clear but yellow, like weak tea
Algae bloom: A rapid increase in algae driven by surplus light and nutrients. In a home tank it shows up either as green water — algae suspended in the water column — or as a coating on surfaces. The lever that works fastest is almost always light duration, not a bottled treatment.Where it mattersThe water has turned green and I cannot see through it · Hard green dots are growing on the glass · Nitrate
Ammonia: The nitrogen compound that fish excrete continuously and that decaying food and plants release. A hobby kit measures total ammonia, which combines the mild ammonium ion and the damaging free ammonia form. How much of that total is dangerous depends on pH and temperature.Where it mattersAmmonia · My new tank is reading ammonia and there are already fish in it · The Nitrogen Cycle
Ammonium: The charged form of ammonia, written NH₄⁺, which is far less harmful to gill tissue than the uncharged free ammonia it trades back and forth with. Cooler and more acidic water favors ammonium; warmer and more alkaline water favors the dangerous form. Your test kit counts both together and cannot tell you the split.Where it mattersAmmonia · pH and KH
Bacterial bloom: A cloud of free-floating heterotrophic bacteria feeding on dissolved organic material, which makes the water look milky white. It is common in tanks under two months old and it clears on its own once the biological filter takes the food supply away. The cloudiness is harmless; whatever ammonia reading is behind it is not.Where it mattersThe water has gone cloudy and white · The Nitrogen Cycle
Beneficial bacteria: The nitrifying bacteria that convert ammonia to nitrite and nitrite to nitrate. They live on surfaces rather than in open water, which is why filter media matters and why water changes do not remove them. Their population sizes itself to the waste stream, so it grows and shrinks with your stocking and feeding.Where it mattersThe Nitrogen Cycle · Nitrite · Filter Media & Cartridges
Biofilter: Also called biological filtration: the colony of nitrifying bacteria living on your filter media, substrate and decor, together with the flow that feeds them. It is the part of a filter that actually processes waste, and the part that a tap-water rinse destroys. Everything else a filter does is mechanical or chemical.Where it mattersThe Nitrogen Cycle · Filters · The filter output has slowed to a trickle
Buffer: The capacity of water to resist a change in pH, supplied in a freshwater tank mainly by carbonates and bicarbonates. Water with a healthy buffer holds its pH steady against the acids that biological processes produce all day. Water with none does not, and its pH can move a long way overnight.Where it matterspH and KH · My pH reads differently every time I test it
Chemical filtration: Removing dissolved substances from water by binding them to a medium — activated carbon, a synthetic resin, or zeolite. It handles things a sponge cannot catch and bacteria do not eat, such as tannins and some dissolved organics. All chemical media are consumables with a working life measured in weeks.Where it mattersFilter Media & Cartridges · The water is clear but yellow, like weak tea
Chloramine: Chlorine combined with ammonia, used by many utilities as a longer-lasting disinfectant residual than free chlorine. It is deliberately stable, so letting water stand does not remove it. When a conditioner splits the molecule, the ammonia half is released into the tank and your test kit will read it.Where it mattersChlorine and Chloramine · Water Conditioners & Additives · My new tank is reading ammonia and there are already fish in it
Dechlorinator: A water conditioner that neutralizes chlorine and, in most modern products, chloramine as well, so that tap water can go into a tank. Dose it by the manufacturer's label for the volume you are adding — this site does not print dosages for any product. Treat every drop of replacement water, including top-ups.Where it mattersWater Conditioners & Additives · Chlorine and Chloramine
Diatom: A single-celled alga with a silica shell, which in an aquarium appears as a brown dust coating glass, plants and substrate. It is characteristic of tanks between about three weeks and three months old and it fades as the tank matures. Wiping and siphoning is the whole treatment.Where it mattersEverything is coated in brown dust
Drip acclimation: Introducing new fish by slowly dripping tank water into the bag or container they arrived in, so that temperature and chemistry change gradually rather than at once. It matters most when the shop water and your water differ noticeably in pH or hardness. It is a handling technique, not a health measure.Where it matterspH and KH · Temperature
Evaporation versus water change: Evaporation removes water and leaves everything dissolved in it behind, so topping up with fresh treated water restores the level without removing anything. A water change removes water and its dissolved contents together, which is why it is the only action that actually lowers nitrate. Topping up is not maintenance; it is replacing what left as vapor.Where it mattersMaintenance Schedule · Water Change Calculator · Nitrate
GH: General hardness: the total dissolved calcium and magnesium in the water, usually reported in degrees or as ppm calcium carbonate. It governs which species are at home in your water and matters to invertebrates and to plants. It is a different measurement from KH, and the two do not have to move together.Where it matterspH and KH
GPH: Gallons per hour, the rate a filter or pump moves water. Manufacturers quote it for a clean, empty unit, so the figure on the box is a ceiling rather than what your filter delivers with media in it. Real flow falls as media loads up and as the intake collects debris.Where it mattersFilters · The filter output has slowed to a trickle
KH: Carbonate hardness, the measure of how much buffering capacity your water has. It is the reading that explains almost every pH problem in a home aquarium and the one most people never take. Test it in the same session as pH, because a pH number alone cannot be interpreted.Where it matterspH and KH · My pH reads differently every time I test it · Test Kits
Mechanical filtration: Physically straining particles out of the water with floss, foam or a pad. It is what makes water look clear, and it does nothing for anything dissolved. Mechanical media clogs and needs rinsing — in a bucket of tank water, never under the tap.Where it mattersFilter Media & Cartridges · The filter output has slowed to a trickle
Nitrate: The end product of the nitrogen cycle in an ordinary freshwater tank. It accumulates between water changes and comes down when you change water, which makes it an honest record of your maintenance habit. Note that hobby kits report the nitrate ion while drinking water standards use nitrate-nitrogen, and the two scales differ by about a factor of four and a half.Where it mattersNitrate · Nitrate is high no matter how often I change water
Nitrite: The intermediate step between ammonia and nitrate, produced by one group of bacteria and consumed by another, slower group. It accumulates when the second group has not caught up, which is the middle and longest stage of cycling. It interferes with oxygen transport in the blood, so it is not a number to leave sitting.Where it mattersNitrite · Ammonia is at zero but nitrite will not come down · The Nitrogen Cycle
Nitrogen cycle: The chain by which bacteria convert fish waste into progressively less harmful compounds: ammonia to nitrite to nitrate. Establishing it in a new tank takes roughly four to eight weeks without seeded media, and it cannot be meaningfully rushed. Half the problems on this site are this process wearing a different hat.Where it mattersThe Nitrogen Cycle · Ammonia · Nitrite
pH crash: A sudden fall in pH that happens when the water's carbonate buffer runs out and there is nothing left to resist the acids being produced. It is not a random event: KH declines for weeks first, which is why a KH test gives you plenty of warning. Nitrification also slows sharply in acidic water, so a crash can stall a tank's own cycle.Where it matterspH and KH · My pH reads differently every time I test it · Ammonia is at zero but nitrite will not come down
Quarantine: Keeping newly bought fish or plants in a separate container for a period before they join the main tank. For plants it is a practical way to avoid importing algae. For animals it is a health practice, and how to run one for a specific species is a question for an aquatic veterinarian rather than for this site.Where it mattersBlack or dark red tufts are growing on my plants and decor
Resin: A synthetic bead or granular medium used for chemical filtration, targeting dissolved organic compounds. It is finer-grained in its action than activated carbon and is usually run in a mesh bag inside the filter. Like all chemical media it exhausts and needs replacing or regenerating on the manufacturer's schedule.Where it mattersFilter Media & Cartridges
Turnover rate: How many times per hour a filter theoretically processes the tank's entire volume, found by dividing the filter's rated gallons per hour by the tank volume in gallons. It is a rough planning figure rather than a specification, because real flow is always lower than the rating. Dead spots matter as much as the number does.Where it mattersFilters · Black or dark red tufts are growing on my plants and decor · Water Change Calculator
Zeolite: A mineral used in some filter media that adsorbs ammonium from the water. It can buy time during an ammonia problem, but it also removes the food supply the nitrifying bacteria are trying to grow on, so it can lengthen a cycle. It saturates and stops working, and it is not a substitute for changing water.Where it mattersFilter Media & Cartridges · Ammonia