“How much water should I change?” is one of the most common questions in fishkeeping, and the answer in litres is simple arithmetic. The harder questions, such as which percentage suits your tank and how often to do it, depend on your fish, plants, filtration and water tests, and are best answered by a specialist shop or club. What this guide offers is the arithmetic: how to convert a percentage into litres, how much old water is left after one or several changes and how to plan the buckets. It also explains why accurate volume matters. The Aquarium Water Change Calculator does the first step for any tank.
Litres to replace
water to replace = tank volume × percentage ÷ 100
| Tank water volume | 10% | 20% | 25% | 30% | 50% |
|---|---|---|---|---|---|
| 60 L | 6 | 12 | 15 | 18 | 30 |
| 100 L | 10 | 20 | 25 | 30 | 50 |
| 200 L | 20 | 40 | 50 | 60 | 100 |
| 300 L | 30 | 60 | 75 | 90 | 150 |
All figures are litres. For gallons, use the same percentage on the volume in gallons: 25% of 50 US gallons is 12.5 gallons. One US gallon is 3.785 litres, and one UK gallon 4.546.
Use the real water volume
The figure that matters is the water in the tank, not the number on the box. Substrate, rock, wood and equipment take up space, and the water level sits below the rim. Our guide to how to calculate aquarium volume shows a tank sold as 142 litres that holds about 109 litres of real water. A 25% change of 142 is 35.5 litres; of 109, it is 27.3. Using the box figure would remove 30% more than you intended. If you can, measure the real volume by filling with buckets.
How much old water is left?
A water change does not replace the water the way emptying a jug does; the new water mixes with what is left. After a change of a fraction f, the share of the original water left is 1 − f.
| Change | Original water left after one change |
|---|---|
| 10% | 90% |
| 20% | 80% |
| 25% | 75% |
| 30% | 70% |
| 50% | 50% |
After several changes of the same size, multiply: (1 − f) to the power n.
| After n weekly changes | 10% each | 20% each | 25% each | 50% each |
|---|---|---|---|---|
| 1 | 90.0% | 80.0% | 75.0% | 50.0% |
| 2 | 81.0% | 64.0% | 56.2% | 25.0% |
| 3 | 72.9% | 51.2% | 42.2% | 12.5% |
| 4 | 65.6% | 41.0% | 31.6% | 6.2% |
| 8 | 43.0% | 16.8% | 10.0% | 0.4% |
This is a mathematical picture of dilution only. It treats the original water as a single block and ignores everything the fish, plants and filter add between changes, so it does not tell you what the water contains at any moment. Its use is to show that repeated changes remove old water gradually and that a large single change does not “reset” a tank any more than steady changes do. It also shows that even eight changes of 25% leave a tenth of the original water.
Plan the buckets
A water change is a logistics task: remove, prepare and add.
- Work out the litres from the real volume and your percentage.
- Choose a bucket of known size, such as 10 litres, and mark it.
- Count the buckets: 25 litres is 2.5 buckets of 10.
- Prepare the new water as your water source and treatment require, and match its temperature to the tank. Ask a specialist shop what your water needs.
- Remove water slowly and avoid disturbing fish and plants.
- Add the new water gently, and check equipment is working afterwards.
For a large tank, use a hose and a dedicated bucket. Keep tools for aquarium use only, and rinse them without soap. Unplug heaters and filters before lowering the water level, and plug them in only when the water is back, following the maker’s instructions.
Planning a weekly schedule
Pick a day, a percentage and a routine, and write it down. A table helps:
| Tank | Real volume | Change | Litres | Buckets of 10 L | Weekly time |
|---|---|---|---|---|---|
| Small | 60 L | 20% | 12 | 1.2 | 15 minutes |
| Medium | 100 L | 25% | 25 | 2.5 | 25 minutes |
| Large | 200 L | 25% | 50 | 5 | 45 minutes |
The times are illustrations. The point is that a larger tank needs more effort for the same percentage, which affects whether you will keep it up. If a routine is too much, it will be skipped, so choose one you can maintain.
Big, small and emergency changes
Percentages differ by situation. Many keepers do smaller, regular changes, and some do larger ones less often. A very large change can alter temperature and water chemistry quickly, which can stress animals; your shop or club can advise on limits for your set-up. In an emergency, such as a leak or a problem identified by tests, get advice at once from a specialist or vet familiar with fish.
Water treatments and the volume to use
Water conditioners and other treatments are measured out by volume according to their labels. The volume used may be the volume of new water added or of the whole tank, depending on the product. Read the label, and use the real volume. If you add treatment to the whole tank, your real water volume matters; if to the new water, the buckets matter. Our guides to aquarium volume and tank size help with the sums.
Common mistakes
- Using the box volume rather than the real volume.
- Not matching the temperature of new water.
- Forgetting to unplug equipment.
- Using the wrong bucket size and miscounting.
- Taking the dilution maths as a water-quality guarantee.
- Skipping the routine because it is too big.
- Not reading the treatment label.
Questions keepers ask
Is it better to change a little often or a lot rarely? The arithmetic of dilution is the same for the total over time: four weekly changes of 25% remove the same share of the water as one change of about 68%, since 0.75 to the power 4 is 0.316. The practical differences, such as stress on the fish and the effect of changes in temperature and chemistry, are for your specialist to advise on.
Do I count the water that is left in the substrate and filter? In the arithmetic, the real water volume is what matters. Some water in the filter and substrate is part of the system and is not removed in a change.
How do I handle a very large tank? Use a hose to drain and refill, a measured container to count and a water treatment method that suits large volumes. Plan the job so that you can finish it, and never leave a tank part-filled.
What about salt water? Marine tanks need prepared salt water at the right salinity, which takes time and equipment. The percentage arithmetic is the same; the preparation is not.
What if I change more than I planned? Tell your specialist, and check temperature and conditions before adding the new water. Do not guess.
A schedule you can copy
| Week | Percentage | Litres (100 L tank) | Buckets of 10 L | Done |
|---|---|---|---|---|
| 1 | 25% | 25 | 2.5 | |
| 2 | 25% | 25 | 2.5 | |
| 3 | 25% | 25 | 2.5 | |
| 4 | 25% | 25 | 2.5 |
Fill in the percentage that your specialist recommends for your tank and stock, and tick each week as you go.
When the sums are not enough
The arithmetic here tells you litres and dilution. It does not tell you what is in your water, how your fish are doing or whether a change is needed today. Water tests, the behaviour and appearance of the fish and the advice of a specialist all matter more than any schedule on paper. Use the numbers to plan and to avoid mistakes, and use your own observation and expert advice to decide what to do. If fish look unwell, ask a specialist or a vet familiar with fish promptly rather than waiting for the next scheduled change.
Quick summary
- Water to replace = tank volume × percentage ÷ 100; 25% of 100 litres is 25 litres.
- The share of old water left after one change is 100 minus the percentage.
- After four weekly 25% changes, about 32% of the original water remains (a dilution picture only).
- Use the real water volume, count buckets and match temperature.
- Choose a routine you can keep, and ask a specialist about your stock.
- Use the Aquarium Water Change Calculator.
Frequently asked questions
How do I calculate a water change?
Multiply the tank's water volume by the percentage and divide by 100. For 100 litres at 25%, that is 25 litres.
How much old water is left after a water change?
The share left is (100 − percentage) %. After a 25% change, 75% of the old water remains.
What happens after several changes?
Multiply the remaining fractions. After four 25% changes, 0.75 to the power 4 is about 32% of the original water.
What percentage should I change?
It depends on the tank, stock and your water tests. Ask a specialist shop or club. This guide gives the arithmetic.
Should I use the box volume or the real volume?
The real volume. Substrate, decor and the gap below the rim reduce it.
How do I carry the water?
Count buckets of known size, and match the temperature of the new water to the tank.
Sources
- Pure arithmetic: litres replaced = volume × percentage ÷ 100; share of original water left = (1 − fraction changed)^n
- NIST Guide to the SI, Appendix B.8 (exact US liquid gallon = 3.785411784 litres)
Related calculators
- Aquarium Water Change CalculatorWork out how much water to replace for the percentage change you choose.
- Aquarium Volume CalculatorWork out your fish tank's volume in litres and gallons from its dimensions.
- Aquarium Filter Turnover CalculatorSee how many times per hour your pump or filter moves the whole tank volume.
Keep reading
- How to Calculate Aquarium Volume (Litres/Gallons)How to calculate aquarium volume from length, width and height in cm or inches: the formulas, US and UK gallon conversions and real water volume.
- Fish Tank Size Chart: Gallons, Litres and WeightA fish tank size chart for common US gallon sizes with typical dimensions, volume in gallons and litres and water weight, plus how to read nominal sizes.
- US Gallons vs UK Gallons for AquariumsA US gallon is 3.785 litres and a UK gallon 4.546, a 20% gap. Convert tank sizes both ways, see why 55 gallons can mean 208 or 250 litres, and fix water sums.
- Round vs Rectangular Tank VolumeHow to calculate round, cylinder, hexagon and rectangular aquarium volume: formulas, a cylinder table for 30 to 60 cm, a footprint comparison and real water volume.