Water Parameters
Six or seven numbers describe almost everything about your water's chemistry — and there is no single "correct" target for most of them. The right target is the one your fish actually need.
Key facts
- Ammonia and nitrite should both read zero in an established tank — essentially universal agreement.
- pH, GH and KH have no single "correct" number — the right target depends on which species you keep.
- KH (carbonate hardness) buffers pH: while the buffer holds, pH stays stable; once it runs out, pH can crash suddenly rather than drift gradually.
The six or seven numbers
- Temperature — obvious, but the right range differs by species.
- pH — how acidic or alkaline the water is.
- GH (general hardness) — dissolved mineral content, mostly calcium and magnesium, which affects how a fish osmoregulates.
- KH (carbonate hardness) — the water's buffering capacity: it neutralizes incoming acids and keeps pH from swinging, until the buffer runs out.
- Ammonia and nitrite — toxic waste products, which should normally read zero.
- Nitrate — the cycle's end product, managed by water changes and plants (fully covered in the nitrogen-cycle guide).
Why there is no single "correct" number
Target pH, GH, KH and temperature are set by the species you keep, not by one universal figure for every tank. Fish from soft, acidic water (many South American species) and fish from hard, alkaline water (many livebearers, rift-lake cichlids) need opposite conditions. The practical approach is to decide what you want to keep first, then match your parameters to that stock — not the other way around.
For KH specifically, sources commonly cite roughly 3-6 dKH as a comfortable range for a typical planted or community tank — as a commonly recommended range, not a strict rule.
Ammonia, nitrite and nitrate — the exception
These three are the rare exception to "no single number": ammonia and nitrite should read zero in nearly any tank. Nitrate has no single ceiling, but the general guidance and why sources disagree on the exact figure are covered in full in the separate nitrogen-cycle guide rather than repeated here.
How often to test
A new or not-yet-cycled tank is commonly tested often — roughly every 3-4 days, or several times a week, during the first several weeks. An established, stable tank is usually fine tested weekly to every two weeks, and always before a water change.