How water changes reduce nitrate
When you remove a fraction of the water and replace it, the concentration moves that fraction of the way toward the level in the new water:
new level = new water + (current − new water) × (1 − change %)
Several changes in a row multiply: after n changes the remaining difference is (1 − change %)n. To reach a target in one change:
change % = (current − target) ÷ (current − new water)
Example
A tank at 40 ppm nitrate with tap water at 5 ppm: a 25% change brings it to 31 ppm. To get to 20 ppm in one go you would need to change about 57% of the water.
Big change or several small ones?
Two 25% changes remove less than one 50% change (they reach 24.7 ppm instead of 22.5 ppm in the example), but they are gentler on fish because temperature and water chemistry shift less at once. For a heavily polluted tank, several moderate changes over a few days is usually the safer route.
Frequently asked questions
How much does a 50% water change lower nitrate?
It halves the difference between the tank and the new water. From 40 ppm with 0 ppm water you reach 20 ppm; with 5 ppm tap water you reach 22.5 ppm.
How often should I change aquarium water?
Many keepers change 20–30% weekly. The right amount depends on stocking, feeding and plants; test nitrate regularly and adjust.
Why does my nitrate stay high after water changes?
Check the nitrate level of your tap water. If the new water already contains nitrate, water changes can never bring the tank below that level.