How often should you change aquarium water?
“25% weekly” is a decent default, not a rule. Your tank tells you the right answer through nitrate — if you are logging it.
Water changes do three jobs: dilute nitrate and dissolved organics, restore minerals and alkalinity, and physically remove debris. Different tanks need different amounts of each. A heavily planted 20-gallon and a goldfish tank of the same volume are not remotely the same maintenance problem.
A starting schedule by tank type
| Tank | Volume per change | Interval | Why |
|---|---|---|---|
| 5-gallon betta or shrimp | 30–50% | Weekly | Small water volume means fast swings. |
| 10-gallon community | 25–30% | Weekly | Standard baseline for light stocking. |
| 20-gallon planted | 25–30% | Weekly–fortnightly | Plants consume nitrate; watch potassium and hardness. |
| Turtle tank | 30–50% | Weekly, plus siphoning | High waste load, debris on the bottom. |
| Goldfish / high-stocking | 50% | Weekly | Nitrate production outruns most filters. |
| Saltwater reef | 10–20% | Weekly–fortnightly | Stability matters more than volume. |
The nitrate maths
If nitrate rises from 10 to 40 ppm in seven days, your tank produces 30 ppm of nitrate per week. A 25% change removes 25% of whatever is in the water — so it removes 10 ppm of that 30, and you end the week higher than you started. Run the arithmetic on your own numbers and the schedule writes itself:
- Target: nitrate under 20 ppm before each change for most freshwater tanks, under 10 for sensitive species.
- If nitrate is still climbing week over week, increase the volume before you increase the frequency.
- Two smaller changes back to back (2 × 25%) beat one large change (50%) when you need to avoid shocking livestock.
Use the water change calculator to turn a current nitrate reading and a target into a percentage — no arithmetic, no guessing.
Signs your schedule is not keeping up
- Nitrate above 20 ppm before each change, consistently.
- pH lower before each change than after it — a sign alkalinity is being consumed.
- Visible mulm on the substrate a few days after siphoning.
- Fish gasping at the surface in the morning, before the lights come on.
- A filter that smells strongly between services.
What to log, and why it pays
Log the volume changed, the date, and nitrate immediately before the change. Three or four weeks of those three data points tell you more about your routine than any forum thread. Set the interval as a recurring reminder so the change happens on the day it is due, not the day you notice the water looks slightly yellow.
Frequently asked
Is a bigger water change always better?
Do I need to change water in a heavily planted tank?
Should I vacuum the substrate every time?
Ammonia spike: what to do in the first 24 hours
A step-by-step response to a positive ammonia reading — what to change, what not to do, and the two numbers that decide whether you need to act tonight.
Read →How to cycle a new aquarium (fishless, day by day)
What actually happens during the nitrogen cycle, how long each stage takes, the readings that tell you where you are, and when it is safe to add fish.
Read →Stop guessing what your tank did last week
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