How much water should I change to lower nitrate?

Changing a fraction f of the water lowers nitrate by that fraction (with nitrate-free tap water). From 40 ppm, a 50% change brings it to about 20 ppm.

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How much water should I change?

Q: Is a big water change dangerous?

A sudden large change in temperature or chemistry can stress fish. Match temperature, dechlorinate, and split very large reductions into changes of at most 50%, a day or two apart.

(N − 20)nitrate over target

(N − tap)what a change removes

% to changeat most 50% at once

Is a big water change dangerous?

A sudden change in temperature or chemistry can stress fish. Match the temperature, dechlorinate, and split big reductions into changes of at most 50%, a day or two apart.

How often should I change water?

In a cycled tank, 20–30% once a week keeps nitrate low for most tanks. Heavily stocked tanks need more.