How much water to change to lower nitrate
Water changes are the usual way to bring nitrate down. Each change removes a share of the nitrate and replaces it with tap water, so the drop is smaller than people expect when the tap water has nitrate of its own. The table starts at 80 mg/L and shows the reading after each of four changes.
| Each change | Tap water | After 1 | After 2 | After 3 | After 4 |
|---|---|---|---|---|---|
| 25% | 0 mg/L | 60 | 45 | 33.8 | 25.3 |
| 25% | 10 mg/L | 62.5 | 49.4 | 39.6 | 32.2 |
| 50% | 0 mg/L | 40 | 20 | 10 | 5 |
| 50% | 10 mg/L | 45 | 27.5 | 18.8 | 14.4 |
Values are mg/L (ppm) in the tank, assuming the new water mixes fully and nothing else adds or removes nitrate between changes.
Why the number is high
- Water changes are too small or too far apart for the amount of fish and food.
- More food goes in than the fish eat. Uneaten food and waste break down into nitrate.
- The tank holds many fish for its volume. Check the real volume with the tank volume calculator, since substrate takes up room.
- The tap water already has nitrate. Test it, because it sets the lowest level a water change can reach.
- Few plants. Fast-growing plants take up nitrate, slow or no plants do not.
What to do
Use the calculator to get the share of water for your target. Very high readings are often brought down over a few changes spread over several days (Aquarium Co-op suggests this too), since big changes in water temperature or chemistry can stress fish. Match the new water's temperature to the tank.
Sources differ on targets. Aquasabi suggests 10–25 mg/L for planted tanks. Aquarium Co-op suggests keeping fish tanks under 80–100 mg/L, and lower for sensitive species. Check what yours need. This page is about the arithmetic and does not replace advice about your particular fish.
Retest a day after the change. If the number comes back up quickly, the cause is feeding, stocking or the interval between changes.