A water change is a dilution
A 30% change removes 30% of tank water and replaces the same volume. The final concentration combines nitrate from the remaining 70% and the new 30%.
Formula: current concentration × remaining ratio + source concentration × replacement ratio.
- Remaining ratio = 1 - change rate
- After change = current × remaining + source × replaced
- Test both samples with the same method
Example: 30% change from 40 ppm
At 40 ppm in the tank, 5 ppm in source water, and a 30% change, 40 × 0.7 + 5 × 0.3 equals 29.5 ppm.
That is higher than the 28 ppm result from zero-nitrate source water because the replacement contributes 1.5 ppm.
- Remaining water contribution: 28 ppm
- Source-water contribution: 1.5 ppm
- Estimated result: 29.5 ppm
Add the rise between repeated changes
Use zero rise for back-to-back changes. For changes days apart, add the average nitrate produced between them by feeding, waste, and filtration.
Repeated changes cannot push the tank below source-water nitrate. Ongoing production also raises the long-run equilibrium.
Do not make abrupt changes from one number
An appropriate target depends on livestock, plants, stocking, and test uncertainty. A large water change can also shift temperature, pH, and hardness.
Use the result to plan, observe livestock and source-water conditions, and split corrections into smaller changes when needed.