Aquarium water change calculator
Enter your water volume and the size of change you want, and see how many litres to take out and put back, how many buckets that is and what repeated changes really achieve. Add a nitrate reading to see how big a change it takes to bring it down.
Your water change
Use the water in the tank, not the size on the box. Work out your water volume.
Enter your water volume to see your result.
Bring nitrate down (optional)
Enter your test results to see how big a change it takes to reach the level you want.
Test your tap water too. Nitrate in the new water limits how low a water change can take the tank.
How the amount is worked out
The water to change is your volume multiplied by the percentage. A 25% change on a 120-litre tank means taking out and replacing 30 litres. Use the water actually in the tank rather than the size on the box, which is usually a fair bit more. The volume calculator works it out.
Why repeated changes do not add up
Three 25% water changes do not replace 75% of the water. Each change only dilutes what the previous one left behind, so the second change removes a quarter of the 75% that remained, not a quarter of the original. After three changes, about 58% of the old water has been replaced.
The share of old water left after several changes is (1 minus the change size), multiplied by itself once for each change. That is why a big problem, such as very high nitrate, is not fixed by a week of routine changes, and why regular changes keep a tank steady instead of catching up after the fact.
Bringing nitrate down
Nitrate builds up between water changes, and a change lowers it in proportion to the water replaced, less whatever the new water brings in. That second part is easy to forget. In the UK and the EU, tap water can legally contain up to 50 mg/L of nitrate, so test yours: if your tap water has 20 mg/L, no amount of water changing will take the tank below 20.
When nitrate is very high, spread the correction over several changes a day or two apart rather than one huge one. Fish that have lived in poor water adjust to it, and a sudden shift in pH and hardness can do more harm than the nitrate.
How much and how often
Most keepers change 10 to 30% of the water every week. Heavily stocked tanks, large messy fish and small tanks benefit from the upper end; lightly stocked, heavily planted tanks can often manage with less. Your test kit is the best guide: if nitrate climbs week after week, change more water or change it more often.
Whatever size you choose, keep it consistent. Regular, similar changes keep the water steady, and that matters more to fish than hitting any single number.
Doing the change
- Treat new tap water with a dechlorinator before it goes in, dosed the way the label asks.
- Match the temperature of the new water to the tank, within a degree or two.
- Vacuum the substrate as you drain, so the waste comes out with the water.
- Refill slowly, pouring against the glass or onto a plate so the substrate is not disturbed.
- In a marine tank, replace with salt water mixed to the same salinity. Fresh water is only for topping up what evaporates.
Common questions
How much water should I change in my aquarium?
Most tanks do well with 10 to 30% a week. Let your nitrate readings guide you: if nitrate keeps rising, change more water or change it more often.
Do three 25% water changes equal a 75% change?
No. Each change dilutes what the last one left, so three 25% changes replace about 58% of the water. Only a single 75% change replaces 75%.
How much water do I need to change to lower nitrate?
Divide the drop you want by the gap between your tank and your tap water. Going from 40 to 20 mg/L with nitrate-free tap water takes a 50% change; if your tap water has 10 mg/L, it takes 67%.
Is a 50% water change too much?
Not if the new water closely matches the old in temperature and chemistry and the tank is well kept. In a tank that has gone a long time without changes, do several smaller changes over a few days instead.
Should I include the sump or filter in the volume?
Include a sump, because the change is a share of all the water in the system. The water inside a canister or hang-on filter is small enough to ignore.
