Aquarium Water Change & Dilution Calculator
Estimate water to replace and simulate how repeated changes affect a dissolved parameter.
US liquid gallons (1 US gal = 3.785411784 L). Switching units converts your entries.
Check the entered values and ranges before calculating.
Optional: enter tank volume to see how much water to replace.
Result
Evolution per change
| Change # | Value after |
|---|
Choose your schedule from measured trends, livestock needs and the quality of replacement water.
Model dilution over repeated water changes
Use the same concentration unit for the tank and replacement water. The model mixes each change completely before the next one. Enter the actual water volume if you also want the amount to replace in US gallons or liters.
Formula and units
After one change: Cnext = Ccurrent × (1 − f) + Cnew × f. After n changes: Cn = Cnew + (Cstart − Cnew) × (1 − f)ⁿ. Here f = change percentage ÷ 100.
Worked example
Start at 40 ppm with replacement water at 0 ppm and change 20% each time. The results are 32 ppm after one change and 25.6 ppm after two. In a 50 US gallon system, each change replaces 10 US gallons.
Assumptions and limits
The model assumes constant volume and no new production, dosing or consumption between changes. Real aquarium trends can differ. Use it for a simple dissolved-concentration estimate; pH is logarithmic and cannot be predicted by directly averaging pH values.
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Frequently Asked Questions
How many gallons should I change?
Multiply actual aquarium water volume by the change percentage divided by 100. A 20% change in a 50 US gallon system replaces 10 US gallons. Volume is optional for the concentration simulation.
Why do several small changes differ from one large change?
Each later change also removes some of the previously added water. With complete mixing, two 20% changes replace 36% of the original water, rather than 40%.
Will the real parameter follow this simulation exactly?
No. The model assumes complete mixing, constant volume and identical replacement water, with no production or consumption between changes. It is suitable for simple dilution estimates, not direct pH prediction or biological processes.
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