Water Change Calculator
Water Change Calculator works out exactly how many changes and what percentage to remove to hit your target parameter — supports any measurable reading.
Calculator
Select a parameter to pre-fill typical values, or enter your own below
Test your tap water or use 0 for RO/distilled
Enter your values above to see results
Make water changes easier
Equipment that simplifies routine maintenance.
Semi-automatic gravel vacuum and water changer — removes substrate debris without lifting buckets
Highly concentrated dechlorinator — neutralises chlorine and chloramine in tap water, detoxifies ammonia during cycling
Monitor ammonia, nitrite, nitrate, and pH — test before each water change to know exactly what you need to reduce
I’ve used everything listed in my own tanks. If you buy through a link, I earn a small commission — at no extra cost to you, and it never affects what I recommend.
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Frequently Asked Questions
- How often should I do water changes in my aquarium?
- For most planted aquariums, a weekly water change of 20–30% is a good baseline. If you use the Estimative Index (EI) fertiliser method, a 50% weekly change is required to reset accumulated nutrients. Heavily stocked fish-only tanks may need twice-weekly changes. The key metric is not frequency but the result — test your water parameters and change enough water to keep nitrate below 20–40 ppm depending on your stocking level.
- How does the water change formula work?
- The calculator uses the dilution formula: change volume = tank volume × (current value − target value) / (current value − source value). This is derived from first principles of dilution chemistry — you're replacing a fraction of tank water with lower-concentration source water until you hit your target. The formula assumes the source water parameter value is lower than your tank's current level (you cannot dilute with water that's equally or more concentrated).
- What is a 'large' water change and why is it a problem?
- The calculator flags changes over 60% of tank volume as large. Very large water changes can stress fish in several ways: sudden shifts in temperature (if the new water isn't matched precisely), changes in water hardness and pH, and the physical turbulence of replacing so much water at once. For sensitive fish or invertebrates, it's safer to split a large reduction into two or three moderate changes over consecutive days, allowing the tank to stabilise between each one.
- Why does source water matter in the calculation?
- Most tap water is not pure — it contains measurable levels of nitrate, GH, KH, and other parameters depending on your local supply. If your tap water has 10 ppm nitrate and your tank has 40 ppm nitrate, a 50% water change won't reduce it to 20 ppm — it will only reduce it to 25 ppm. Entering your actual source water value (test your tap water to find it) gives you an accurate result. Use 0 for RO water or distilled water, which has no dissolved minerals.
- Can I use this calculator for parameters other than nitrate?
- Yes — the calculator works for any parameter that follows dilution chemistry: nitrate, nitrite, ammonia, GH (general hardness), KH (carbonate hardness), TDS, or any other measurable concentration. Select a preset from the quick-select buttons to pre-fill typical values, or enter your own current, target, and source values. The only requirement is that your target must be lower than your current reading, and your source water must have a lower value than your current tank level.