Skip to content
Fish Disease Identifier

Dilution and target planning

Aquarium Water Change & Dilution Calculator

Enter real tank volume, the current test result, a target, and the tested replacement-water result. See whether the target is mathematically reachable and how repeated changes compound.

Free to use. Runs locally in your browser. No account required.

Model a target water change

Use the actual water volume and tested values from both the aquarium and replacement water. All concentration fields must use the same unit and chemical basis.

What are you modeling?
Test results and target

Unit: mg/L or ppm. A blank replacement-water test is not the same as a tested zero.

Staged-change preview

Choose one repeated-change size. The result assumes the tank is refilled to the same volume and mixed between changes.

This calculator does not model pH, dissolved oxygen, medication doses, or the cause of a bad result. Test and condition replacement water, match temperature and other relevant chemistry, and retest after mixing.

Result

Enter tested tank and replacement-water values. The calculator will show whether the target is mathematically reachable, the one-change volume, and a staged projection.

How it works

What the dilution estimate does—and does not—show

The model treats a water change as a simple mixture. It is useful for planning concentration changes, but it cannot decide whether a particular change is biologically safe or solve the source of a bad result.

One perfectly mixed water change

The estimated result is current concentration × the water left behind, plus replacement-water concentration × the fraction added. This is why replacement water cannot be assumed to contribute zero.

  • New value = current × (1 − change fraction) + replacement × change fraction.
  • All test results must use the same unit and chemical basis.
  • A target outside the current-to-source range is unreachable with that water.
  • A 100% modeled change equals the replacement-water value.

Repeated changes compound

A second 25% change acts on the concentration left after the first change. The projection assumes the tank returns to the same volume and mixes fully between steps, with no new waste production or biological removal.

Important limits

Do not use linear dilution for pH, dissolved oxygen, medication dosing, or the apparent ammonia effect of a binder. For ammonia or nitrite, the number is only a snapshot; investigate filtration, source water, recent maintenance, and fish distress.

  • Match temperature and relevant water chemistry.
  • Treat chlorine or chloramine correctly.
  • Maintain aeration and flow during urgent situations.
  • Retest after the aquarium has mixed.

FAQ

Common questions

How much does a 25% water change reduce nitrate?

If the replacement water tests at zero nitrate and the tank is perfectly mixed, one 25% change leaves about 75% of the starting nitrate. If replacement water already contains nitrate, the reduction is smaller.

Why does a second 25% change not remove another 25% of the original value?

Each change removes 25% of the concentration present at that moment. With zero-nitrate replacement water, two separate 25% changes leave about 56.25% of the original value, not 50%.

What if replacement water already contains nitrate or ammonia?

Enter the tested replacement-water value. The calculator includes its contribution and will warn when the requested target cannot be reached with that source water. Do not treat an untested blank as zero.

Can this calculator be used for pH?

No. pH is logarithmic and depends on buffering, gas exchange, and chemistry that a linear dilution formula cannot represent. The model also excludes dissolved oxygen and medication dosing.

Does a water change fix the cause of high ammonia?

Not necessarily. Dilution can lower a measured value temporarily, but it does not repair a filter failure, overstocking, decaying material, chlorine exposure, or another continuing source. Test again and investigate the cause.

Is the calculated water change a recommendation?

No. It is a mathematical estimate. Water-change safety depends on source-water treatment, temperature and chemistry matching, livestock, tank type, and how stressed the fish already are.