FishBaseHQ

2026-10-28 · 9 min read

How often to change aquarium water

Most stocked community tanks want a weekly 25% water change. Goldfish want more. Test nitrate and dose conditioner for the new water, not the whole tank.

Default: about 25% once a week on a stocked freshwater community. That is a starting point, not a religion. Top-offs for evaporation are not water changes — they put minerals back and leave nitrate where it was.

The dose is the water change calculator. Conditioner for the gallons you add, not the whole tank. The longer habit list is the water change guide and the maintenance schedule.

Adjust from tests, not vibes

What you seeWhat to do
Nitrate climbing past 40 ppm between weekly 25%sMore volume, more often, or less livestock
Nitrate stuck at 5–20 with plants and light stockingYou can sometimes stretch to 10 days. Test.
Goldfish, overstocked cichlids40–50% weekly is normal, not a failure
Ammonia or nitrite above 0 with fish in the tankChange now. Then figure out the cycle or the filter.

A schedule you will actually keep beats a heroic 80% change after three weeks of neglect.

What “a water change” means

  1. Match temperature within a couple of degrees.
  2. Dechlorinate the new water. Chloramine needs a conditioner that handles it, not a splash of “dechlor” from 1998.
  3. Vacuum mulm, not every grain of sand every time. Corys live in that sand.
  4. Do not replace all filter media on the same day.

If fish gasp after a fill, you either skipped conditioner or shocked them with temperature. Gasping.

New tanks

During a cycle you may change water to dump ammonia. Livestock is not supposed to be in there yet. Cycling. Clear water is not a cycle.

Planted tanks still need changes. Plants use nitrate; they do not void the siphon. Estimative Index tanks often plan a large weekly change on purpose — that is a fertilizing method, not a reason to never siphon a community.

How many gallons is 25%

A 20 long that actually holds ~18 gallons of water: about 4.5 gallons out. A sticker “55” that measures 48×13×21 is closer to 56 rim-full, less after substrate. Measure: volume. Then dose the replacement water, not 55 gallons of conditioner into a bucket of 12.

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