Tool
Water change calculator
Turn a percentage into litres, so you know exactly how much to take out and put back.
Your result
About 25 litres per change
A 25% weekly change on this tank is roughly 25 L each time.
What this calculator does
Water changes are the single most useful maintenance habit in fishkeeping: they dilute waste, replenish minerals and keep water stable. Most guidance is given as a percentage — "change 25–30% a week" — but you still need to know how many litres that is. This tool converts the percentage into a volume so you can mark buckets or set a routine.
When to use it
- Setting a weekly routine you can measure.
- Working out a bigger change after a problem.
- Planning how much dechlorinator to add.
How to use it
- Use the real water volume, allowing for substrate and décor.
- Enter the percentage you intend to change.
- Measure out that much at the next change, using a marked container so you repeat it easily.
Worked example
A 100-litre tank on a 25% weekly change replaces 25 litres each time. If you would rather do two smaller changes instead, 12.5 litres twice a week gives a similar, gentler effect and is easier on sensitive fish.
Typical frequencies
| Situation | Typical change |
|---|---|
| Lightly stocked, well filtered | 20–25% weekly |
| Average community tank | 25–30% weekly |
| Heavily stocked or messy fish | 30–50% weekly |
| New or cycling tank | Small, frequent changes while it settles |
Limitations
- Match temperature and treat the water. Replace with dechlorinated water close to tank temperature.
- Bigger is not always better. Very large changes can shock fish if the new water differs much in chemistry.
- Filtration still matters. Water changes support a working filter; they do not replace one.
Frequently asked questions
Should I change water every week?
Weekly changes suit most tanks. The exact volume depends on stocking and filtration; consistency matters more than hitting an exact figure.
Can I change too much water?
You can, if the replacement water differs sharply in temperature or chemistry. For a problem like an ammonia spike, several moderate changes are safer than one huge one.
Do I need to gravel vacuum every time?
Not necessarily. Vacuum part of the substrate on a rotation so you clean the whole tank over several weeks without disturbing it all at once.
Related guides
Sources and research notes
Percentages are typical community guidance; the right volume depends on stocking, feeding and filtration.