💧 Water Change Calculator
Enter your tank volume and the percentage you change to see exactly how much water to remove — and how much dechlorinator to add to the fresh water you put back.
🧮 Plan Your Water Change
What is a Water Change Calculator?
It takes the guesswork out of routine maintenance. Enter your tank's volume and the percentage you want to swap, and it tells you exactly how many gallons to siphon out — then how much water conditioner to add so the fresh tap water going back in is safe for your fish.
Regular partial water changes are the backbone of a healthy aquarium: they dilute nitrate the filter can't remove, replenish minerals, and keep water chemistry stable. Getting the volumes right means a consistent routine, correctly dosed conditioner, and far less stress on your fish.
❓ Frequently Asked Questions
How much water should I change and how often?
A common routine is a 10–25% water change every week for a stocked freshwater tank, though heavily stocked or planted tanks may need more. Enter your volume and chosen percentage here to see the exact gallons to remove. Consistent, moderate changes keep nitrate in check better than occasional large ones.
How much dechlorinator (water conditioner) do I add?
Dose the conditioner for the amount of new water you're adding, not the whole tank, unless the label says otherwise. This calculator defaults to a common 5 ml per 10 gallons, but brands differ — set the field to match your bottle. Always treat tap water to neutralise chlorine and chloramine before it reaches your fish.
Should I match the temperature of the new water?
Yes. Add fresh water at roughly the same temperature as the tank to avoid shocking your fish. A few degrees is usually fine, but a large swing — especially cold water into a warm tank — stresses fish and can trigger disease. Mix hot and cold at the tap or let a bucket acclimate before adding it.
Why are regular water changes so important?
The nitrogen cycle converts fish waste into nitrate, which the filter can't remove — only water changes do. Left to build up, nitrate stresses fish and fuels algae. Partial changes also replenish minerals and dilute dissolved organics, keeping water chemistry stable. They're the single most effective piece of routine tank maintenance.