Aquarium water parameters explained
AquaLog guide · Published
In short: In an established freshwater aquarium, ammonia and nitrite should read 0 ppm, nitrate should stay below about 20–40 ppm, and temperature and pH should suit your fish. Stable readings matter more than chasing an exact pH.
What should each parameter read?
| Parameter | Target for most freshwater tanks | Why it matters |
|---|---|---|
| Ammonia | 0 ppm | Toxic. Any reading means the tank isn't cycled, is overstocked, or something has died. |
| Nitrite | 0 ppm | Toxic. It stops the blood carrying oxygen. |
| Nitrate | Below about 20–40 ppm | Low toxicity, but high levels stress fish over time and feed algae. |
| pH | Suited to your species; often 6.5–7.5 | Sudden swings stress fish more than a steady value outside the ideal. |
| GH (general hardness) | Suited to your species | Calcium and magnesium. It matters most for soft- or hard-water species and shrimp. |
| KH (carbonate hardness) | Often 3–8 dKH | Buffers pH. Very low KH lets pH crash. |
| Temperature | Most tropical fish: 24–27 °C (75–81 °F) | Sets metabolism and how much oxygen the water holds. |
Ammonia and nitrite: why should they be zero?
Both come from fish waste and rotting food, and both are removed by filter bacteria in a cycled tank. Ammonia becomes more toxic as pH and temperature rise. If you see either above zero, test again, do a partial water change, feed less, and look for a cause: a new tank, too many new fish at once, a dead fish or snail, or a filter that has been cleaned too hard. See how to cycle an aquarium.
Nitrate: how high is too high?
Nitrate is the end product of the nitrogen cycle and only leaves through water changes and plants. Many keepers aim for under 20 ppm; most freshwater fish tolerate up to about 40 ppm. Steadily rising nitrate is the clearest sign your water change schedule needs adjusting.
pH, GH and KH: what do they mean?
pH measures acidity. GH counts dissolved calcium and magnesium; KH counts carbonates, which buffer pH. Different fish come from different waters. Neon tetras come from soft, acidic blackwater, while livebearers such as mollies prefer hard, alkaline water. Every species page in our care guide gives its pH and hardness range. Choose fish that suit your tap water rather than fighting it with chemicals, and change it slowly when you must.
Shrimp are especially sensitive to hardness, because they need minerals to moult. Cherry shrimp are among the easiest.
How often should you test?
- New tank or new fish: every few days until readings are stable.
- Established tank: every one to two weeks, ideally before a water change.
- Any time fish act strangely: gasping, clamped fins, hiding or not eating. Test first, then act.
What about reef and marine tanks?
Saltwater adds more to watch. Typical reef targets are specific gravity 1.024–1.026, alkalinity 7–11 dKH, calcium 380–450 ppm and magnesium 1250–1400 ppm, with ammonia and nitrite at zero and nitrate low. Fish-only systems are more forgiving.
One reading tells you little; the trend tells you a lot. AquaLog keeps every test on a chart per tank so slow drifts stand out. Start logging your water.