Aquarium water-chemistry glossary
Plain-language definitions of the water-chemistry and cycling terms that show up across freshwater aquarium content.
New to water chemistry? Read in this order
- Nitrogen cycle — the foundation. Everything below is a stage of this cycle.
- Ammonia → nitrite → nitrate — the three readings on every test kit, in the order bacteria convert them.
- pH / GH / KH — the source-water chemistry that decides which species fit your tap.
- Water change — the routine that resets all of the above. Pair with the water-change calculator for per-session math.
Nitrogen cycle
The biological process where bacteria in a tank's filter convert toxic ammonia → nitrite → less-toxic nitrate. A tank is 'cycled' when this conversion happens fast enough to keep ammonia and nitrite at zero.
Ammonia
A nitrogen compound (NH₃/NH₄⁺) released by fish waste and decaying matter. Acutely toxic to fish at concentrations above ~0.25 ppm. In a cycled tank, ammonia should always read 0.
Nitrite
Nitrogen compound (NO₂⁻) produced when ammonia-eating bacteria break down ammonia. Toxic to fish; converted to less-toxic nitrate by a second bacterial group. In a cycled tank, nitrite should read 0.
Nitrate
Nitrogen compound (NO₃⁻) — the end product of the nitrogen cycle. Far less toxic than ammonia or nitrite. Removed primarily by water changes and live plants. Conservative target: under 40 ppm.
pH
A 0-14 scale measuring how acidic or basic water is; 7 is neutral. Most freshwater aquarium fish tolerate 6.5-8.0. Stability matters more than hitting a specific target.
GH (general hardness)
Total dissolved calcium and magnesium in water, measured in dGH (German degrees) or ppm. Soft water = low GH (<5 dGH); hard water = high GH (>12 dGH). Most community fish tolerate 5-15 dGH.
KH (carbonate hardness)
Carbonate / bicarbonate concentration in water — sometimes called alkalinity. Buffers pH against drops. Higher KH = more pH stability. 4-8 dKH is typical for community freshwater tanks.
Water change
Replacing a portion of tank water with fresh dechlorinated water. The first routine maintenance task most new tanks depend on. Typical schedule: 25-30% weekly for most freshwater community tanks.