Aquarium Water Change Calculator | Litres to Replace for Any Tank and Rate
Enter your tank volume and change rate to see the exact litres to replace and the water left behind. No more multiplying in your head over the bucket every weekend.
💡 About this tool
"Do a 25% water change" sounds simple until you have to work out what that is for your actual tank. For odd volumes like a 60L setup, doing the math in your head is where most people over- or under-drain.
Drop in your volume and the percentage and you get both numbers at once: the amount to take out (a 60L tank at 25% is 15L) and what stays behind (45L). Slide the rate from 10% to 25% to 50% and watch the litres move, so you can settle on a routine before you ever touch the siphon. It is handy when you are dialing in maintenance for a freshly cycled tank, or running several tanks on different schedules.
🧐 Frequently Asked Questions
How much water should I change each week? A common guide is 10–25% per week, and many beginners with a community tank land around 25–30%. Heavily stocked or lightly planted tanks lean higher; lightly stocked tanks lean lower. Let your nitrate readings set the final number.
Is it safe to change a lot at once? Swapping more than 50% in one go can swing pH, temperature and mineral levels enough to stress fish, so it is usually avoided. This tool only does the arithmetic — it does not judge what is safe for your setup.
Why not just replace all the water? A full change flushes the beneficial bacteria and resets the nitrogen cycle, which can crash the tank. Repeated partial changes are the standard approach.
Can I use gallons instead of litres? The tool calculates in litres, but it is really just taking a percentage of whatever number you enter. Type a value in any unit and read the result back in that same unit.
What is the "water remaining" figure for? It is the old water still in the tank after you drain. Knowing it helps you estimate water chemistry once the fresh top-up mixes in.
📚 Why partial changes beat a full reset
The point of a water change is mostly to dilute nitrate — the end product of the nitrogen cycle (ammonia to nitrite to nitrate) that your filter cannot break down further. Nitrate only comes down when you physically remove and replace water, which is why hobbyists do small frequent changes instead of one big reset. Stirring up the glass and gravel just before you siphon lifts trapped waste so it leaves with the old water. If you are refilling from the tap, a dechlorinator is typically added to neutralise chlorine before or as the new water goes in.