Aquarium Water Change Calculator
Weekly aquarium swap volume and bucket count from tank size.
What it does
Weekly water changes dilute what filters miss. Nitrate climbs daily, and a steady swap keeps fish steadier than rare big flushes.
The tool above turns tank plus share into buckets. A 20 gal tank at 25 % needs 5 gal each week, or 1 bucket. The metric match is 76 L toward 19 L.
Share follows stocking. Light planted tanks may cruise at 10 %, typical community tanks sit at 25 %, and heavy or problem tanks may need 50 % until tests settle.
Technique guards the swap. Water near tank temp, conditioner dosed per label, and a slow pour protect bacteria, plants, and fry far more than exact gallons.
Siphoning beats topping off. Waste leaves with the drain water, while top offs only replace evaporation and let nitrate climb.
How to use
- Pick imperial (gal) or metric (L).
- Enter tank size: 20 gal or 76 L suits many starters.
- Pick share: 25 suits most tanks.
- Read change volume first.
- Read bucket count next.
- Siphon, refill, and condition weekly.
Formula
- tank
- Tank volume.
- share
- Share changed each week: 10, 25, or 50.
Buckets are 5 gal imperial and 19 L metric, the same volume each way. Timing stays weekly in both modes.
Worked example
Example: 20 gal at 25 %
Imperial defaults: 20 gal tank at 25 %. These are also the default values, so the page opens on this answer.
- Tank: 20 gal.
- Share: 25 %.
- Change: 20 x 0.25 = 5 gal.
- Buckets: 5 / 5 = 1 bucket (5 gal).
- Timing: every week.
- Tool shows 5 gal, 1 bucket (5 gal), Every week.
Reference table
| Tank | 25 % | Buckets |
|---|---|---|
| 10 gal | 2.5 gal | 0.5 buckets (5 gal) |
| 20 gal | 5 gal | 1 bucket (5 gal) |
| 40 gal | 10 gal | 2 buckets (5 gal) |
| 76 L | 19 L | 1 bucket (19 L) |
Limits
- Volume math only. Tests still rule frequency.
- New cycling tanks follow kit guidance, not defaults.
- Salt and meds need separate dosing.
- Large shares need temp matched refill.
Common mistakes
- Topping off instead of changing.
- Swapping 50 % weekly in a light tank with no need.
- Pouring cold water straight from the tap.
- Washing filter media in tap water.
- Chasing pH with each change.
Try it yourself
Quick checks. Answers sit tucked under each question.
10 gal at 25 %?
10 x 0.25 = 2.5 gal. Buckets are 2.5 / 5 = 0.5 buckets (5 gal).
40 gal at 50 %?
40 x 0.50 = 20 gal. Buckets are 20 / 5 = 4 buckets (5 gal).
Questions
How much water to change?
Multiply tank by the share. A 20 gal tank at 25 % needs 5 gal each week.
How many buckets?
Divide change by bucket size. Here 5 / 5 = 1 bucket.
10, 25, or 50 %?
Weekly 25 % suits most stocked tanks. Light planted tanks may hold 10 %, heavy or cycling tanks may need 50 % per guidance.
How do I refill safely?
Match temp within a degree or two and dose conditioner for the full tank per label. Pour slowly to spare roots and fry.
Vacuum or top off?
Siphon the gravel as you drain. Waste leaves with the water, which a top off alone never removes.
Does this dose salt?
No. Salt, meds, and cycling need their own plans.