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Aquarium water change calculator

Estimate how repeated partial water changes affect a measured concentration, using net water volume, replacement water concentration and any increase between changes.

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01

Your inputs

The starting values are an example.

The actual amount of water; excluding the volume of substrate and equipment.
A single selected, measured dissolved substance.
The same substance, in the same unit of measurement.
Your own scenario, not a recommended change ratio.
No addition before the first change. 0 = pure dilution example.

Decimals: 12.5 · Thousands: 1,234.56. Follow the field instructions for lists.

02

Your result

Enter your values and select Calculate.

Your calculation will appear here.

Your result will include the relevant values and units.

Understand the calculation

Formula and example

How it is calculated

C after = (C before + increase) × (1 − fraction replaced) + replacement concentration × fraction replaced. The increase is zero before the first change.

Worked example

100 l, 50 mg/l, 5 mg/l replacement water: after three 25% changes, 23.984375 mg/l, with no intermediate growth.

Things to check

  • The calculation does not indicate a safe concentration and does not provide advice on fish care or water change frequency. Species, water parameters, and measurement reliability are separate issues.
  • Use data for the same dissolved substance in the same mg/l unit. Plant uptake, decomposition, and regeneration are represented only by your own concentration increase between changes. Evaporation is not included in the model.

Frequently asked questions

Why isn't the removed fraction three times 25%?

On the second and third occasions, you also remove water previously added. With complete mixing, the proportion of original water is multiplied by 0.75 each time.

What does replacement water concentration mean?

The measured mg/l value of the same dissolved substance in the water being added. If this is higher than the value measured in the aquarium, the change may increase the calculated concentration.