Aquarium water-change calculator
Work out how much water to change to hit a target parameter — or how a planned change will shift NO₃, PO₄, GH or KH given your tap water. Pure dilution math, no guesswork.
After the change
New value
29.5 mg/L
Water to change
30 L
A 30% change blends 40.0 (tank) with 5.0 (tap) proportionally.
Quick reference
| Change | New value (mg/L) | Litres |
|---|---|---|
| 10% | 36.5 | 10 L |
| 25% | 31.3 | 25 L |
| 33% | 28.4 | 33 L |
| 50% | 22.5 | 50 L |
| 75% | 13.8 | 75 L |
| 100% | 5.0 | 100 L |
Log this water change in AquaPilot
Track each change and the parameters before/after — the app builds the full maintenance history automatically.
How it works
- 1
Pick the parameter and volumes
Choose what you're adjusting (NO₃, GH, KH…), enter your tank volume and the current level you measured.
- 2
Tell the calculator about your tap water
Measure or look up your tap value for the same parameter. Dilution can never push below the tap level.
- 3
Pick a percentage — or a target
Either enter a planned % and see the resulting value, or enter the target you want and read the required % and litres.
When to use it
Lowering nitrate or phosphate
Classic case: NO₃ crept to 40 mg/L, you want 10 mg/L. The calculator tells you whether a 50% change is enough or if you need two rounds.
Softening or hardening the tank
GH or KH drifting? See how a planned change — pure tap, RO-mix, or remineralised water — will shift them.
Shrimp and wild-type fish
Stable parameters matter more than big changes. Use the quick table to plan small regular changes that don't shock sensitive livestock.
How aquarium dilution actually works
A water change is linear blending: you remove X% of the tank's water (carrying X% of the parameter) and replace it with the same volume of tap water. The new level is a weighted average of what stayed and what came in.
The formulas
Both directions are just algebraic rearrangements of the same mixing equation. Units cancel — use the same unit on both sides (mg/L, °dH, etc.).
Typical schedules
Rough targets you'll see in hobbyist communities. Adjust to your stocking, feeding and testing cadence.
- Lightly stocked / low-tech10–20% / week
- Standard community tank20–30% / week
- EI-dosed planted tank50% / week
- Shrimp & soft-water tanks10–15% / week
Things the formula doesn't model
Pure dilution math is a good baseline, but a real water change has side effects. Plan for these to avoid surprises.
Match temperature
New water should be within ±1 °C of the tank. Cold shocks stress fish and can crash beneficial bacteria.
Dechlorinate
Municipal tap water usually contains chlorine or chloramine. Use a conditioner (Seachem Prime, API Tap Water Conditioner) or aged water.
RO / deionised water
If tap is harder than you need, blend RO with tap to hit target GH/KH, or remineralise RO with a salt mix before adding.
Don't overdo a single change
Above ~50% in one go can trigger osmotic shock in sensitive fish and invertebrates. Prefer two smaller changes 24 h apart.
Worked examples
Common scenarios with real numbers, using tap water values typical of European cities.
| Scenario | Result |
|---|---|
| 100 L tank, NO₃ at 40 mg/L, tap 5 mg/L — how much to get to 15 mg/L? | ~71% change (~71 L) |
| 200 L tank, KH at 10 °dKH, tap 6 °dKH — after a 30% change? | New KH ≈ 8.8 °dKH (60 L changed) |
| 60 L shrimp tank, TDS at 350, tap 250 — weekly 15% change? | New TDS ≈ 335 (9 L changed) |
| 120 L tank, NO₃ at 60 mg/L, tap 30 mg/L — can I get to 10? | Not with tap water — tap is above target. Use RO. |
Frequently asked questions
Why can't I dilute below the tap-water value?
Because the water you add has that value. Even a 100% change leaves you at the tap level — it's the asymptote. To go lower, use RO or deionised water that's effectively zero.
Is this accurate for GH and KH?
Yes — GH and KH are conservative properties (they don't decay in the short term), so pure dilution applies. pH is not: it buffers nonlinearly, so don't use this tool for pH.
How often should I do water changes?
Depends on bioload, lighting and plant mass. A common baseline: 20–30% weekly for a stocked community tank; 50% weekly for high-light EI tanks; 10–15% for shrimp and low-tech setups.
Can a big water change hurt my fish?
Mismatched temperature, un-dechlorinated tap, or a >50% change that swings GH/KH/pH suddenly can stress sensitive livestock. Match temp, dechlorinate, and split large changes over 24 h.
Does the calculator account for fertilizer or salts I'm dosing?
No — it only models dilution. If you dose after the change, add your dose to the new value to get the post-dose level. AquaPilot's full app tracks both automatically.
Related tools
Want AquaPilot to do this automatically?
Log your water changes and measurements, and the app will plan the next change before the parameter drifts.
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