How often to change aquarium water
Most guides say 25% a week. That fits some tanks and is too little or too much for others, because nitrate builds up at a different speed in every tank. Measure how much yours rises in a week, and the schedule follows from it.
How to measure the rise
Right after a water change, test nitrate. Test again a week later without changing water. The difference is your weekly rise in mg/L (ppm). If the tank read 15 after the change and 25 a week later, the rise is 10.
The math
A change of share f swaps that part of the tank for tap water, so the reading becomes N × (1 − f) + tap × f. Then nitrate climbs by the rise R before the next change. After a few rounds the pattern repeats: just before each change the reading sits at tap + R ÷ f.
To keep that peak at or under your limit, the change must be at least R ÷ (limit − tap). With a rise of 10 mg/L a week, clean tap water and a 40 mg/L limit, that is 10 ÷ 40 = 25% a week. Longer gaps need bigger changes, since the rise adds up in the meantime.
Longest gap between changes
Days between changes that keep the tank at or under 40 mg/L, with tap water at 0 mg/L. The table follows the limit and tap values in the form above.
| Nitrate rise per week | 20% changes | 30% changes | 50% changes |
|---|---|---|---|
| 5 mg/L | 11.2 days | 16.8 days | 28 days |
| 10 mg/L | 5.6 days | 8.4 days | 14 days |
| 20 mg/L | 2.8 days | 4.2 days | 7 days |
Assumes the rise is steady and the new water mixes fully. A value above 7 days means a weekly change has room to spare.
Worked example
A tank reads 15 mg/L after a change and 25 a week later, so the rise is 10. The tap water has 0 and the limit is 40. A 30% change every week settles between 23.3 and 33.3 mg/L: 33.3 × 0.7 = 23.3, plus 10 is 33.3 again. That leaves room to skip a week, which would put the reading near 43.
The same tank on 20% weekly changes settles at 50 before each change, over the limit. With tap water at 10 mg/L and 30% changes the peak is 10 + 10 ÷ 0.3 = 43.3, over the 40 limit, so the changes need to be larger or more frequent.
What other guides say
Practical Fishkeeping says about 25% once a week works well for most people but depends on your tank, and suggests comparing tank and tap nitrate: a gap of more than 40 ppm means more water, more often.
Aquarium Co-op considers nitrate generally below 40 ppm safe for most fish, prefers 30% changes, and sets the schedule from how fast a tank gains nitrate. Their example tank gains about 10 ppm a week and needs a change every week.
Sensitive species and planted tanks may need a lower limit, so check what yours need. This page covers the arithmetic only. For a single high reading, see nitrate too high, and for the exact amount in liters or gallons, use the water change calculator.