How many liters do you need to change? Straight from a percentage or from your current and target nitrate – no registration needed.
Routine changes: 20–30% per week is a solid default for most community tanks.
Nitrate target: The required change fraction is 1 − (target ÷ current). The closer target and current are, the smaller the change can be.
Split large changes: Doing over 50% at once can stress sensitive animals with sudden parameter swings – better to spread it across two smaller changes over a few days.
Nitrate rises again afterward: The calculation applies to the moment right after the change – fish and filter bacteria keep producing nitrate continuously.
Instead of recalculating every time: your tank volume is saved, every water change is logged, and you see the nitrate trend over time instead of just a snapshot.
Log every change. Always see the last change date and upcoming reminders.
Log NO₂ and NO₃ after every change – and watch the trend over weeks.
Never forget a water change again – automatic reminders on your own schedule.
Work out net liters from your tank dimensions – the basis for this calculator.
Mixing ratio of RO/DI and tap water for your next water change.
How many fish can your tank handle? Check stocking density based on net volume.
Liters, gallons, °C/°F, GH/KH to ppm – every aquarium unit in one place.
Change fraction = 1 − (target nitrate ÷ current nitrate). Example: current 40 mg/l, target 20 mg/l → 1 − (20/40) = 50% water change needed.
20–30% per week is typical for most community tanks. Heavily stocked or lightly planted tanks benefit from more frequent or larger changes, while calm, densely planted or aquascape tanks often need less.
A single very large water change (over 50%) can stress sensitive fish and shrimp due to sudden shifts in water parameters. If nitrate is high, it's better to spread it over two smaller changes a few days apart instead of one huge change.
The formula applies to the moment right after the change. After that, your fish and filter bacteria keep producing new nitrate, so the value rises again until the next water change.
Create your tank, track water changes and test results – free, no download.
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