Water Change Calculator
"Change 25–30% of the water every week" is good advice — but how much water is that, actually? Enter your tank size and percentage, and get gallons, litres, and bucket counts.
Quick reference: weekly changes for common tanks
| Tank | 25% change | 30% change | 50% change |
|---|---|---|---|
| 10 gal | 2.5 gal (9.5 L) | 3 gal (11.4 L) | 5 gal (18.9 L) |
| 20 gal | 5 gal (18.9 L) | 6 gal (22.7 L) | 10 gal (37.9 L) |
| 29 gal | 7.3 gal (27.5 L) | 8.7 gal (32.9 L) | 14.5 gal (54.9 L) |
| 55 gal | 13.8 gal (52.1 L) | 16.5 gal (62.5 L) | 27.5 gal (104.1 L) |
Three habits that make water changes work
- Match temperature and treat the water. New water should be close to tank temperature and always treated with a dechlorinator — chlorine and chloramine in tap water harm fish and the filter bacteria you're relying on.
- Siphon the gravel, don't just skim. Waste settles into the substrate. A gravel vacuum during the change removes the source of tomorrow's ammonia, not just today's nitrate.
- Little and often beats big and rare. A steady weekly 25–30% keeps water chemistry stable. Emergency 50%+ changes are for genuine problems — ammonia spikes, contamination — because big swings stress fish too.
Not sure how much your tank needs? Your stocking level decides it — run your fish list through the stocking calculator and it will tell you the weekly percentage your plan requires.