Water Change Calculator for Nitrate

A water change removes a share of whatever is dissolved in the water, then adds replacement water. If you swap a fraction f of the water, the new reading is:

new = old × (1 − f) + replacement × f

With RODI the replacement reading is 0, so a 25% change leaves 75% of the nitrate. With tap water that has 10 ppm of nitrate, the replacement term adds a little back, and it sets a floor: you can never get below the nitrate in the new water.

Each change removes a share of what is left, not a fixed amount. Three 25% changes do not remove 75%. They leave 42% of the starting nitrate, because each change works on a smaller number. Even a 50% change only halves the reading.

To bring a high reading down, you can do a larger change if the tank can take it, or repeat smaller ones a few days apart. Large changes shift salinity, temperature and pH all at once, so match the new water to the tank first.

It assumes the water is perfectly mixed and nothing is added or consumed between changes. In a real tank, fish keep producing nitrate and algae, plants and bacteria consume it, so the reading starts rising again as soon as you finish. Rock and substrate also hold some nitrate and release it slowly.

Use the numbers as a plan, then test a day or two after the change to see where the tank really landed. Count the sump and plumbing in your system volume.

The same sum applies to phosphate, TDS, and any other dissolved substance with the same unit for all three readings. It does not apply to things the tank produces or uses continuously, like alkalinity in a reef tank that is being consumed by corals.