Regular water changes remove excess nutrients, dilute waste, and replenish essential minerals. This calculator gives you a rough estimate of how much water to change each week so fish and plants thrive. Over time, you can adjust the schedule based on water test results and the appearance of your tank.
The recommended percentage starts at 20% weekly. We increase it if your aquarium has a heavy bio-load or few plants, and we decrease it slightly if your filtration is efficient. The equation is simple:
Multiply the result by your tank volume to get the amount of water to replace each week. If your calculated percent falls below 10%, we still recommend at least a small change to maintain stability. Higher waste levels or a crowded tank may require more frequent changes.
You can enter your tank size in gallons or liters. Select the appropriate unit from the dropdown and the calculator converts everything automatically. After calculating, use the Copy Result button to keep the recommendation handy on your phone when performing maintenance.
| Tank | Waste | Filter | Change % |
|---|---|---|---|
| 30 gal | 2 | 2 | 20% |
| 55 gal | 3 | 1 | 30% |
For a moderately stocked 30-gallon tank with average filtration, about 6 gallons should be replaced weekly. A heavily stocked 55-gallon tank with basic filtration might need closer to 30% changed to keep nitrate levels low.
Siphon debris from the substrate during water changes and clean filter media in old tank water to preserve beneficial bacteria. Test ammonia, nitrite, and nitrate regularly to refine your routine. Stability is key—sudden large water changes can shock fish, while skipping maintenance may lead to algae or disease.
Consistent water care helps aquatic plants grow vigorously and supports delicate fish species. Alongside water changes, avoid overfeeding, trim dead leaves promptly, and ensure adequate water flow. By using this calculator and observing your aquarium’s response, you’ll create a reliable schedule that keeps your underwater ecosystem flourishing.
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