Water change forecast
See GH, KH, NO₃ and TDS after a partial water change — before you drain and refill.
For comparable readings over time and with this calculator, keep conditions consistent:
- NO₃ — after a change, wait 15–30 min for mixing, then test.
- GH — like KH; for water changes, measure tap/replacement water and tank separately.
- KH — more stable than pH; test before a water change or 15–30 min after mixing.
- TDS — let water mix after a change; tap and RO water differ greatly.
- After topping up, wait 15–30 min for the filter to mix, then test GH, KH, NO₃ or TDS.
Change volume
In the tank now
Replacement water
Tap water (typical): GH 8–15, KH 4–10, NO₃ 0–40, TDS 150–400 ppm — varies by region.
RO / distilled: GH and KH near 0, NO₃ 0, TDS 0–20 ppm — remineralize (GH+/KH+) before adding.
Target mix: enter tested GH/KH of the bucket, then check the forecast.
Cafter = Ctank·(1 − f) + Creplace·f
f = W / T
- Cafter
- parameter value after the change (GH, KH, NO₃ or TDS)
- Ctank
- current value in the tank
- Creplace
- value in replacement water
- T
- tank volume, L
- W
- volume removed and replaced in one change, L
- f
- change fraction W / T
The calculator shows where the water should land after mixing if tank and replacement readings are correct. It does not replace a test kit — verify key parameters after the change.
- 15–30 min after refilling, GH, KH, NO₃ and TDS are usually close to the forecast once the filter has mixed the water. No need to wait hours.
- Accuracy depends on your inputs — test tap, RO or bucket water separately. Tap NO₃ sets the floor after changes.
- TDS and GH/KH are guides — salt mix differs; do not argue over 1 ppm.
- The tank is alive — NO₃ and PO₄ shift over days; the forecast is the mix moment only.
- Gravel vacuuming can release NO₃ — planning uses the reading before the change.
- Match temperature within ±1–2 °C, dechlorinate, avoid sharp GH/KH jumps (see warnings). When in doubt, change less.