Free tool
Water-parameter interpreter
You tested the water — now what do the numbers mean? Enter your readings and get a plain-English read on whether your fish are safe, and exactly what to do next. Because a dying fish is nearly always a water problem: test, don't guess.
Safe freshwater target ranges
The targets this tool checks your readings against. Enter your own numbers above for a plain-English verdict and next steps on your water.
| Parameter | Target | Why |
|---|---|---|
| Ammonia (NH₃) | 0 ppm (RSPCA: < 0.1) | Anything above 0 is toxic and burns gills. |
| Nitrite (NO₂) | 0 ppm (RSPCA: < 0.2) | Toxic too — above 0 is a problem. |
| Nitrate (NO₃) | Under 50 ppm (aim ≤ 20–40) | Above 50 stresses fish and feeds algae — do a water change. |
| pH | ≈ 6.0–8.5 | Stability matters more than the exact number; don't chase it with chemicals. |
| Temperature | Tropical 23–27 °C · Coldwater 17–24 °C | Match your fish; check the heater/thermometer if outside the band. |
Ammonia/nitrite/nitrate targets follow RSPCA welfare guidance. These are targets, not a diagnosis — test with a reliable liquid kit.
Water-parameter questions
What are safe water parameters for a freshwater aquarium?
Ammonia and nitrite should both read 0 ppm — recognised welfare guidance (RSPCA) puts ammonia under 0.1 and nitrite under 0.2, and any reading above 0 is toxic. Keep nitrate under 50 ppm (lower is better; ≤20–40 is ideal). pH is best somewhere around 6.0–8.5 and, above all, stable. Tropical tanks run about 23–27 °C, coldwater tanks about 17–24 °C.
Is my nitrate too high?
Under about 20 ppm is nice and low, and up to 40 is fine if you keep up regular water changes. From 40 to 50 ppm it's getting high — time for a water change. Above 50 ppm is too high: fish are stressed and algae is fed, so do a larger or more frequent water change and check for overstocking and overfeeding.
My ammonia or nitrite is above 0 — what should I do?
Any ammonia or nitrite above 0 is harmful, so do a water change now to bring it toward 0. A reading above 0 also means the tank isn't (or is no longer) fully cycled — if it's new, you're mid-cycle: protect the fish with water changes, feed lightly, and read the nitrogen-cycle guide before adding any more fish.
What pH is best for community fish?
Most community fish are content anywhere in roughly the 6.0–8.5 range, and stability matters far more than hitting an exact number. Don't chase a target with chemicals — match your fish to the water you have and keep it steady. Low carbonate hardness (KH under 3 °dKH) lets pH swing, so watch stability there.