Aquarium water changes: how often and how much
Updated
A regular water change is the cheapest and most effective piece of tank maintenance. It removes dissolved organics, resets accumulated nutrients and keeps parameters stable.
How much and how often
- planted tank with CO₂ and strong light: 30–50 % once a week,
- planted tank without CO₂: 20–30 % once a week to once every two weeks,
- new tank with aquasoil: more often, see cycling,
- heavily stocked with fish: more and more often.
Consistency matters more than the exact percentage. One big change a month shifts conditions in a jump; smaller weekly changes keep them calm.
Why change water that looks clean
Clarity says little about water quality. Nitrate, phosphate and dissolved organics build up even when the water looks great. With the Estimative Index (EI) fertilizing method a weekly change of around 50 % is part of the method – it removes the surplus so nutrients do not accumulate.
Temperature, chlorine and hardness
- new water should be within 1–2 °C of the tank,
- treat tap water with a conditioner that binds both chlorine and chloramine,
- keep hardness consistent: if you mix RO with tap water or remineralize, do it the same way every time,
- for shrimp it is worth measuring the TDS of the new water and comparing it with the tank.
Combining it with other maintenance
The best order is: trim plants and remove floating debris first, wipe the glass, then drain water while siphoning dirt from the substrate surface and from corners where it collects. Do not dig into aquasoil, just skim above it.
Do not clean the filter at every change. When the flow drops, rinse the media in water drained from the tank, not under the tap.
What to log
The date and percentage, optionally the temperature and TDS of the new water. When algae or plant problems appear you can then see whether the water change rhythm changed – that is one of the first things worth checking.