A water change is one of the simplest jobs in fishkeeping, but it is often surrounded by rigid rules. Change exactly this percentage. Do it exactly once a week. Vacuum every grain of substrate. Replace filter media while you are there.
A healthier way to approach the job is to understand what the water change is actually doing. Once you know that, the percentage becomes a response to the aquarium rather than a number copied from somebody else’s tank.
What a water change removes
Fish waste and food eventually become dissolved compounds in the water. Nitrate is the one beginners hear about most often, but it is not the only thing that accumulates. Dissolved organics, phosphate and other by-products change over time as the aquarium is fed and lived in.
A partial water change exports some of that material and replaces it with fresh source water. In many systems it also replenishes minerals and buffering compounds that are gradually used or diluted.
That is why a tank can need a water change even when the glass looks spotless.
How much water should you actually change?
There is no universal percentage that is correct for every aquarium. A lightly stocked planted aquarium with controlled feeding may remain stable with a modest weekly change. A heavily stocked cichlid aquarium or fry grow-out system may benefit from much larger or more frequent changes.
For many ordinary community aquariums, a regular partial change in the region of 20–40% is a practical starting point, adjusted according to stocking, nitrate, source water and the stability of the tank. The point is not the exact number. The point is that maintenance should keep the system consistently safe rather than allowing waste to accumulate until a dramatic clean-out is required.
If your aquarium is new, read The Complete Guide to Cycling Your Aquarium as well, because water changes and cycling interact differently depending on whether fish are already present.
Match replacement water before it enters the aquarium
Tap water normally needs to be treated with a suitable conditioner where chlorine or chloramine is present. Temperature should be reasonably close to the aquarium so that the fish are not subjected to a sudden swing.
In areas where source water changes seasonally, or where borehole and municipal water are used interchangeably, it is useful to know the basic chemistry of what is going back into the tank. South African source water can vary substantially from one area to another, so local testing is more useful than assuming somebody else’s pH or hardness applies to your home.
Aquarium Water Parameters Explained for Beginners covers pH, GH, KH and the other numbers that help you understand that source water.
Substrate cleaning should remove waste without sterilising the floor
A gravel vacuum is useful because it allows you to remove water and detritus at the same time. In open gravel, push the siphon into sections of substrate and allow heavier material to settle while lighter waste is removed.
In planted tanks, be more selective. Dense root systems, nutrient-rich substrates and delicate carpeting plants do not benefit from aggressive deep vacuuming. Clean exposed areas and remove obvious debris while leaving healthy planted zones largely intact.
The aquarium does not need to look as though it has just come out of the packaging after every water change. Mature surfaces are part of the ecosystem.
Do not deep-clean the filter on the same day unless you need to
Filter maintenance and water changes are often bundled together because both sound like “cleaning”. The problem is that the filter contains a large amount of the aquarium’s biological filtration.
If flow has dropped, rinse sponges or reusable mechanical media gently in a bucket of removed aquarium water. Preserve established biological media and avoid replacing all of it at once.
Our filter guide explains which parts of a filter are there to trap dirt and which parts are primarily there to support bacteria.
Should you do a water change after overfeeding?
Often, yes. If a large amount of food has entered the aquarium and cannot be removed easily, a partial water change and siphoning can reduce the amount left to decay. The same principle applies after a fish dies unnoticed or another event adds a sudden organic load.
Do not wait for the scheduled maintenance day when there is an obvious water-quality reason to act. A routine exists to support the aquarium, not to prevent you from responding to it.
Large water changes are not automatically dangerous
A well-matched large water change can be very useful. The risk comes when replacement water differs sharply in temperature, pH, hardness or other chemistry from the aquarium, especially if the tank has been neglected for a long time.
If a badly neglected aquarium has drifted far from its source water, several smaller changes may be gentler than one enormous correction. By contrast, a well-maintained aquarium that receives frequent changes can often tolerate substantial water replacement because the old and new water are already similar.
Water changes do not replace good stocking and feeding
If nitrate rises rapidly every week, the answer may not simply be “change more water forever”. Look at the cause. Is the tank heavily stocked? Is too much food entering the system? Is there trapped waste in the substrate? Has filter flow dropped?
The stocking guide and feeding guide help you reduce the amount of waste being created in the first place.
Use the water change as an inspection opportunity
Maintenance is when you are closest to the aquarium. Look at each fish. Check fins and body condition. Notice whether a plant is melting, whether a heater has moved, whether the filter intake is clogged or whether a snail population has suddenly exploded.
Those observations are often more valuable than the water change itself because they allow you to catch a small problem while it is still easy to fix.
A good routine should feel boring
Healthy aquarium maintenance is repetitive in the best possible way. The fish behave normally. The water values are predictable. The filter continues flowing. The water change becomes a calm reset rather than an emergency response.
For a practical weekly rhythm that combines water changes with the other small jobs that keep a tank healthy, use A Simple Aquarium Maintenance Routine That Actually Works. If you are still building the tank from scratch, return to the Ultimate Beginner’s Guide to Aquarium Success.
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