Aquarium Water Change Calculator
| Tank | 25% weekly | Conditioner drops* | Siphon minutes |
|---|---|---|---|
| 10 gal | 2.5 gal | ~10-20 drops | ~5 min |
| 20 gal | 5 gal | ~20-40 drops | ~8 min |
| 55 gal | 14 gal | ~1.5 capfuls | ~15 min |
The water change rule is a quarter of the tank, once a week: a 20-gallon aquarium wants a five-gallon siphon every weekend, a 55-gallon wants fourteen - and the conditioner goes in at the rate the bottle prints, a drop or two per gallon of the NEW water, not the whole tank.
Bottom line: small and often beats big and rare - the 25 percent weekly change removes nitrates without moving the chemistry, while the once-a-month 80 percent panic change swings temperature, pH and chlorine hard enough to kill the fish it was meant to save. The gravel vacuum does two jobs in the same minutes: water out, waste out.
The honest part: the change schedule is the aquarium's whole filtration story told slowly - filters polish the water, but only the change removes the nitrate the filter made. The tank that never gets changed grows algae on the glass and fish gasping at the top, and the owner who changes a quarter every Sunday never meets either problem.
How to use
- Enter tank gallons and read the weekly change volume at 25 percent - siphoned from the gravel, not poured off the top.
- Match the replacement water temperature within a degree or two, and treat it with conditioner at the bottle's rate before or as it enters the tank.
- Set the siphon schedule: same day every week, 15 to 25 minutes for most tanks, and filter media rinsed in the removed tank water - never tap.
Frequently asked questions
How often should I change the water in my aquarium?
Once a week at 25 percent is the standard rhythm for a stocked community tank - 5 gallons out of a 20, 14 out of a 55. Heavily stocked tanks run twice a week at 25 percent, lightly stocked planted tanks stretch to 10 percent weekly, and the nitrate test is the tiebreaker: above 20 to 40 ppm means the schedule is behind the bioload.
How much water conditioner do I need?
The bottle's rate per gallon of NEW water only - a 5-gallon change needs 5 gallons' worth of drops, not the whole tank's. Most concentrates run a drop per gallon or a capful per 10, and overdosing dechlorinator is mostly harmless but pointless; the conditioner's job is done the second the chlorine is neutralized.
Why did my fish die after a big water change?
The chemistry swung: a 70 to 80 percent change resets temperature, pH and chlorine to tap-water values in minutes, and fish that lived in the old chemistry get shocked by the new one - the change that was meant to rescue them was the insult. The safe ceiling is 25 to 50 percent, temperature-matched, conditioned, and spread over hours if the tank is in trouble.
Do I need to remove the fish during a water change?
No for the routine quarter change - the siphon works around them and the fish tolerate it better than a net chase. Everyone comes out for the deep clean, the aquascape rebuild or the medication treatment, in a bucket of their own tank water, because the net chase is itself the stress the change was avoiding.