Enter the actual working water volume in litres, the percentage removed and replaced, and measured concentrations of the same dissolved substance in the tank and replacement water in mg/L. Working volume means water present, allowing for displacement by substrate and decorations.
The calculator assumes you remove water first, then replace the same volume and mix completely. Outputs are replacement volume in litres and expected final concentration in mg/L. This is an ideal mixing estimate, not a dosing calculation or a recommendation about livestock, suitable concentrations or change frequency.
Accepted inputs: volume 0.01–1,000,000 L; change 0–100%; each concentration 0–1,000,000 mg/L. These are numerical entry limits, not recommended aquarium ranges. Volume and concentration values use scalar arithmetic with their physical units stated in the labels; no unit conversion is performed.
OpenStax Chemistry 2e, section 3.3 explains concentration, volume and conservation of solute during dilution. The equation above is a mass-balance derivation for this calculator: remaining solute plus replacement-water solute, divided by final volume. It extends the dilution principle to replacement water with a nonzero concentration.
| Working volume | Change | Old / new concentration | Replacement volume | Expected final concentration |
|---|---|---|---|---|
| 100 L | 25% | 40 / 5 mg/L | 100 × 0.25 = 25 L | 40 × 0.75 + 5 × 0.25 = 31.25 mg/L |
| 200 L | 50% | 20 / 0 mg/L | 200 × 0.50 = 100 L | 20 × 0.50 + 0 × 0.50 = 10 mg/L |
| 80 L | 25% | 10 / 30 mg/L | 80 × 0.25 = 20 L | 10 × 0.75 + 30 × 0.25 = 15 mg/L |
| 100 L | 0% | 40 / 5 mg/L | 0 L | 40 mg/L |
| 100 L | 100% | 40 / 5 mg/L | 100 L | 5 mg/L |
The tank is uniformly mixed before draining; replacement restores the original working volume; the dissolved substance neither reacts nor gains or loses mass through other processes during the change. The result describes concentration after complete mixing. It does not model simultaneous draining and filling, filtration, biological activity, evaporation or later concentration changes.
Use measurements for the same substance and reporting basis in both inputs. Do not mix mg/L reported as an element with mg/L reported as a compound. Do not enter pH: this linear concentration model does not average logarithmic readings.
At 0% the concentration is unchanged. At 100% the mathematical result equals the replacement-water concentration. Between those limits it lies between the two input concentrations; replacement water with a higher concentration raises the result. These endpoints describe the model and are not water-change recommendations.
Displayed values are rounded to three decimal places; that does not establish measurement accuracy. An unavailable result indicates a calculation or input problem and must not be read as zero.
Formula reference
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