Aquarium water change calculator
Every water change removes the same share of nitrate. Enter your test results to see how many changes it takes to reach a target and where a weekly routine settles.
The maths of a water change
A change of p percent replaces that share of the tank with tap water: new nitrate = old × (1 − p) + tap × p. A 50% change halves the nitrate (if your tap has none); a 25% change only removes a quarter of it. To go from 80 ppm to 20 ppm takes two 50% changes, or five 25% changes.
Where a weekly routine settles
If nitrate rises by r ppm a week and you change p each week, the tank settles between a peak of tap + r ÷ p just before the change and a low of tap + r × (1 − p) ÷ p just after. With a 10 ppm weekly rise, 25% changes hold the tank between 30 and 40 ppm; 50% changes hold it between 10 and 20 ppm.
Large changes safely
- Match temperature within 2 °F (1 °C) and always add dechlorinator for chlorine and chloramine.
- If nitrate is very high, spread the reduction over several days: fish adapted to poor water can be shocked by a sudden change in pH and hardness.
- Gravel-vacuum while you drain to remove the waste that will otherwise become nitrate.
See the water change schedule for how much to change in light, heavy, planted and shrimp tanks, the water change guide for how often to change water in different tanks, and the dosing calculator to dose conditioner for the new water.
FAQ
How much water should I change in my aquarium?
Most tanks do well with 20–30% weekly. Heavily stocked tanks, goldfish and cichlids often need 40–50% a week to keep nitrate under 40 ppm.
How many water changes to lower nitrate from 80 to 20 ppm?
Two 50% changes, three 40% changes or five 25% changes, assuming nitrate-free tap water.
Is a 50% water change safe?
Yes for most fish if temperature is matched and the new water is dechlorinated. If nitrate has been high for a long time, make smaller changes over several days.