How to Do an Aquarium Water Change

A step-by-step routine for partial water changes: why they matter, how much and how often, siphoning the substrate, temperature-matching and dechlorinating new water, and why you should not over-clean the biofilter.

Why water changes matter

In a closed aquarium, the nitrogen cycle turns fish waste and leftover food into ammonia, then nitrite, then nitrate. Ammonia and nitrite are consumed by your biofilter, but nitrate is the end product that simply accumulates — and along with other dissolved organics it builds up until you physically remove it. Partial water changes are how you export that nitrate and dissolved waste, take out particulate debris, and top up trace elements and buffers that get used up between changes. Left unchecked, rising nitrate feeds algae and stresses the fish.

How much and how often

For most tanks a routine partial change of about 25–50% per week works well, with roughly 25–30% a sensible starting point; heavily stocked or heavily fed tanks need the larger, more frequent end, while lightly stocked or planted tanks can often go longer between changes. New and high-tech planted tanks usually need larger, more frequent changes at first — for example around 50% weekly for the first several weeks — until they stabilise. Adjust to what your nitrate test actually shows rather than following a fixed number blindly.

Step by step

1. Turn off and unplug the heater, filter and other electricals. If the water line will drop below the heater, let it cool before it is exposed to air so it does not crack.
2. Prepare the replacement water: treat tap water with a dechlorinator to neutralise chlorine and chloramine, and bring it to the same temperature as the tank.
3. Using a gravel vacuum / siphon, draw out your target percentage while hovering over the substrate to lift out settled waste and uneaten food.
4. Do not deep-clean the whole biofilter at the same time. If filter media needs rinsing, rinse it gently in old tank water — never under hot or chlorinated tap water — so you keep the nitrifying bacteria alive.
5. Refill slowly with the temperature-matched, dechlorinated water to avoid a sudden temperature or chemistry swing.
6. Restore flow, plug the heater and filter back in, and confirm the temperature on your thermometer.

Sources: en.wikipedia.org , dennerle.com , www.2hraquarist.com