Water parameters: what do temperature, pH, GH and KH mean?
2 min read · Last updated October 16, 2026
Learn to read the ranges on species pages, interpret your test kit results and change parameters safely.
On every species page you will see temperature, pH and hardness ranges. These values describe the water the species lives in in nature and the limits within which it stays healthy in an aquarium. The most important rule is this: Keeping a value stable is more important than hitting the target exactly. Most farm-bred fish adapt to wide ranges; the real harm comes from sudden change.
Temperature (°C)
Fish are cold-blooded; their metabolism and immunity depend on water temperature.
- Most tropical fish are comfortable at 24–27°C.
- Goldfish and some shrimp want cooler water, such as 18–24°C.
- Discus and rams want 28°C and above.
Use a heater and a separate thermometer; do not blindly trust the heater's setting. The aquarium weather card on the site warns you when there is a heat wave or cold snap in your city.
pH
This is the degree of acidity or alkalinity of the water. 7 is neutral, below that is acidic and above it is alkaline. The scale is logarithmic: pH 6 is ten times more acidic than pH 7.
- Amazon species (neon tetra, discus) evolved in soft, acidic water.
- Malawi cichlids and livebearers (molly, platy) like hard, alkaline water.
Changing pH quickly with chemicals is one of the worst things you can do to fish. If it needs to be changed, do it by natural means (driftwood, leaves, coral sand) and in a way that takes days.
GH (general hardness)
This is the amount of calcium and magnesium dissolved in the water; it is measured in dGH (German degrees).
| dGH | Water |
|---|---|
| 0–4 | very soft |
| 4–8 | soft |
| 8–12 | moderately hard |
| 12–18 | hard |
| 18+ | very hard |
In Türkiye, tap water varies greatly by region; in many cities it is moderately hard to hard. For species that want soft water, reverse osmosis (RO) water or rainwater can be used.
KH (carbonate hardness)
This is the water's capacity to keep pH stable (buffer). If KH is low (below 2 dKH), pH can drop suddenly and fish can be harmed. For most aquariums, 3–8 dKH is a safe range.
Other measurements
- Ammonia and nitrite: Should always be 0. If not, the aquarium is not cycled or it is overstocked.
- Nitrate: Keep it below 40 mg/L; below 20 mg/L is better for shrimp and sensitive species.
Choosing a test kit
Test strips are quick but crude. Liquid drop tests are more reliable. If you are new to aquariums, you should at least be able to measure ammonia, nitrite, nitrate and pH.