Aquarium Water Changes: How Much, How Often, and Why

Water changes are the most boring part of fishkeeping and the one that separates tanks that run for years from tanks that lurch between problems. Twenty minutes a week.

What a water change actually removes

In a cycled tank, ammonia becomes nitrite and nitrite becomes nitrate. That's where the chain stops. Nitrate is far less toxic than either of the first two, and it accumulates continuously, and a standard filter does nothing about it.

Plants take up some. A heavily planted tank genuinely needs fewer water changes for this reason. In a lightly planted or bare tank, the only export is you, with a bucket.

Water changes also dilute dissolved organic compounds, replace trace minerals the fish and plants have used up, and stabilise pH by restoring carbonate hardness. That last one is the sneaky benefit: as KH depletes over weeks, pH becomes unstable, and unstable pH is much harder on fish than a steady value at the wrong number.

How much, how often

Small tanks need more discipline, not less. There's less water to dilute anything that goes wrong, so a nano tank swings faster than a 200-litre one.

The actual procedure

1. 1Unplug the heater. A heater exposed to air while running can crack, and in older units it's a genuine fire and shock risk. Give it a few minutes to cool before draining if you're taking the level down far.
2. 2Siphon from the substrate, not just from open water. Most of the waste is in the gravel, and pushing a gravel vacuum into it pulls out detritus the filter never caught.
3. 3Take out your target volume. Mark a bucket once with a permanent pen so you're not guessing every week.
4. 4Refill with dechlorinated water at close to tank temperature. Within a degree or two.
5. 5Pour slowly, onto a plate or your hand, so you're not blasting the substrate and stressing the fish.
6. 6Plug the heater back in.

The mistakes that cause real damage

Water changes are hard to get badly wrong, and there are four ways people manage it.

• Forgetting dechlorinator. Chlorine damages gill tissue directly. Add it to the new water before it goes in, or dose the tank for the full volume as you refill.
• Temperature shock. Cold refills stress fish and, in some species, trigger disease outbreaks. Ich frequently follows a careless water change.
• Enormous emergency changes. Going from a badly neglected tank to 90 percent fresh water swings pH, hardness and temperature all at once. Do 25 percent daily for a week instead.
• Changing filter media on the same day as a big water change. Both disturb the bacteria. Space them apart, and only ever replace part of the media at a time.

Do I need to change water in a planted tank?

Yes, less often. Plants consume nitrate and some organics, so a densely planted, lightly stocked tank can run comfortably on a 20 percent change fortnightly. That's earned by the plants doing the export, and it's still worth testing nitrate to confirm rather than assuming.

Tanks running injected CO2 and fertiliser dosing usually go the other way and change more water, weekly and larger, to reset nutrient levels precisely.

Set a day, mark the bucket, keep the dechlorinator next to it. The tanks that stay healthy are the ones where this is boring.