See what a water change will actually do to nitrate, phosphate, alkalinity, or any other parameter, and how big a change you'd need to reach a target. It's straight dilution math, so it assumes the tank doesn't add or use anything during the change.
The formula
After a change of fraction f, the new level is: tank value × (1 − f) + new water value × f. To reach a target in one change, f = (tank − target) ÷ (tank − new water). Repeated changes of the same size multiply: three 10% changes leave 0.9 × 0.9 × 0.9 ≈ 73% of the original difference, slightly less effect than one 30% change. Randy Holmes-Farley works through the same dilution logic for reef aquaria in Randy Holmes-Farley, "Water Changes in Reef Aquaria," Reefkeeping Magazine.
Test your freshly mixed salt for alkalinity, calcium, and magnesium so the "new saltwater" values are real. Our water change guide covers matching parameters, and the nitrate and phosphate balance post explains why zero isn't the goal.
FAQ
How much does a 20% water change lower nitrate? If the new saltwater has no nitrate, a 20% change cuts nitrate by 20%: 20 ppm becomes 16 ppm. The tank keeps producing nitrate, so the real number drifts back up between changes.
What size water change do I need to hit a target? Fraction to change = (current − target) ÷ (current − new water value). Going from 40 to 20 ppm nitrate with zero-nitrate salt water needs a 50% change, which is large; several smaller changes are gentler.
Do water changes fix low alkalinity? Only toward your salt mix's value. If your mix is 7.5 dKH and the tank is 6.5 dKH, a 20% change only lifts it to about 6.7 dKH. Use the dosing calculator for real corrections.