How Often to Change Aquarium Water: UK Guide for Beginners

Changing your aquarium water is the single most important job in fishkeeping, and it's also the one beginners get wrong most often. Some people leave a tank untouched for months because the water looks clear. Others panic and replace everything at once, which shocks the fish and does more harm than good.

I've kept tanks for over eight years and reviewed more aquarium gear than I can count, so let me say it plainly: the water change is what separates a tank that thrives from one that slowly poisons its fish. This guide covers how often to change aquarium water in a UK home, exactly how much to take out, and the step-by-step routine I run on my own planted tank.

Why Water Changes Matter

The filter converts toxic ammonia into nitrite and then into nitrate, which is far less harmful. But nitrate doesn't disappear on its own. It builds up until it feeds algae, slows fish growth and leaves your tank looking tired. Water changes are the only practical way to keep nitrate in check, because they physically remove it from the system.

Tap water does a second job that most people miss. It replaces the minerals and carbonate hardness that fish and plants slowly use up, which keeps your pH stable. A tank that never gets fresh water drifts slowly towards acidic, soft conditions that stress fish even when the water looks crystal clear. You can't see this happening, which is why the routine has to be mechanical rather than visual.

The Simple Rule for Most UK Tanks

Start with 25-30% once a week and adjust from there. That's the default that works for the average community tank of 40-100 litres with a normal amount of fish. Weekly means the same day every week, not whenever you remember, because the whole point is keeping the chemistry steady rather than swinging it.

Three situations change the schedule:

• Lightly stocked or heavily planted tanks - nitrate stays low, so 25% every 10-14 days is plenty. Let the test kit confirm it.
• Heavily stocked tanks, or fish that eat a lot - goldfish are the classic example. They produce waste like machines, so 30-40% twice a week keeps things safe.
• Small nano tanks under 20 litres - the water volume is so small that everything swings fast. A 25% change twice a week is the safer minimum.

Little and often beats big and rare every time. A 30% change every week keeps the water stable, while a single 60% change every two weeks makes the chemistry yo-yo, and fish hate yo-yos.

When to Change Water More Often

There are four situations where the weekly rule gets suspended:

• Cycling a new tank - during the 4-8 week nitrogen cycle, change 20-25% twice a week. It keeps ammonia and nitrite spikes survivable without starving the filter bacteria.
• Treating sick fish - many medications break down water quality fast. Daily 25% changes during a treatment course, always with fresh conditioner, give the fish the best chance.
• Algae bloom - green water or slimy walls means nutrients are too high. Change 30-50% every other day, cut the light to 6-8 hours, and feed less until it clears.
• After medicating - when the treatment finishes, change 30% to remove the chemical residue before you judge whether the fish actually recovered.

In a UK summer heatwave, warmer water holds less oxygen and fish produce more waste, so don't be shy about adding an extra change on top of your normal routine.

How to Change Aquarium Water Step by Step

Once you've done this a few times it takes about twenty minutes. Here's the routine I use on my own tank.

Step 1: Switch off the filter, heater and lights. The water level drops during a change, and a heater left running out of the water can crack or overheat. The filter can also run dry.

Step 2: Get your gear together. You need a clean bucket that has never touched cleaning chemicals, a gravel siphon, and your water conditioner. A basic AKORD siphon vacuum does the job perfectly for tanks up to 100 litres, and it's cheap enough to replace when the rubber seals wear out. If you'd rather not squeeze to start the flow, the Horsky squeeze-free siphon starts itself, while the Hygger cleaner with a hose clamp gives you flow control for smaller tanks.

Step 3: Siphon the gravel from end to end. Push the siphon tube into the gravel and work it in slow circles. The suction lifts debris out from between the stones while the heavier gravel falls back down. Work through the whole tank, not just one corner, because waste settles everywhere. This is the step people skip, and it's the step that actually keeps the substrate from turning into a sludge factory.

Step 4: Clean the glass while the water is low. Use an algae pad or a cloth on the inside of the panes. Getting to the glass is much easier when you're not fighting the water level, and it saves you a second trip.

Step 5: Refill with fresh, conditioned water. Fill your bucket from the tap, add the conditioner while the bucket fills so it mixes properly, and match the temperature to the tank by feel or with a thermometer. Seachem Prime is the conditioner I use everywhere - one 100ml bottle treats 4,000 litres, it removes chlorine and chloramine instantly, and it detoxifies ammonia and nitrite in an emergency. If you want the budget option, API Tap Water Conditioner does the same job for less per bottle, though you'll buy it more often. Pour slowly onto a plate or your hand so you don't blast the substrate around.

Step 6: Switch the filter, heater and lights back on. Check the heater dial shows the same temperature as before, and give the filter a couple of minutes to push the bubbles out.

Step 7: Test the water with your liquid kit. Half an hour after the change, run the ammonia, nitrite and nitrate tests. The API Freshwater Master Test Kit gives you over 800 tests, covers pH as well, and is far more accurate than test strips. Write the nitrate result down somewhere.

Step 8: Keep a simple weekly log. Two lines in a notebook per week - date, nitrate level. After a month you'll know exactly how fast your tank builds nitrate, and you can set your own schedule instead of following a rule of thumb.

How Much Water to Change Each Time

The percentages matter because water chemistry is about ratios, not litres. Here's how to read them:

• 10-15% - weekly top-up for lightly stocked or heavily planted tanks. Fine as maintenance, not enough to rescue a dirty tank.
• 25-30% - the standard weekly change for most tanks. This is your default for a standard community tank.
• 40-50% - for rescuing a neglected tank or after a disease outbreak. Only when there's a clear reason to do more.
• Over 50% - emergency only, such as an ammonia spike or accidental poisoning. Anything bigger disturbs the filter bacteria and shocks the fish.

If you're ever unsure, do 25% and test the water a day later. The test result tells you the truth faster than any opinion.

How to Clean Your Filter Without Killing the Good Bacteria

Your filter is a home for billions of beneficial bacteria, and tap water kills them stone dead. So never rinse filter media under the tap, no matter how gross it looks.

Instead, clean the filter every three to four weeks by squeezing the sponges and rinsing the media in a bucket of water you just siphoned out of the tank. Clean one half of the media at a time so the other half keeps the cycle alive. Leave the ceramic rings and bio-balls almost alone - a gentle swirl is all they need, because they're pure bacteria housing. If you've had to do a deep clean and want to rebuild the colony fast, a bottle of API Quick Start adds live bacteria straight back into the tank.

Common Water Change Mistakes to Avoid

• Skipping it because the water looks clear - clear water and healthy water are different things. Nitrate is completely invisible to the naked eye.
• Using a bucket that's touched cleaning chemicals - even a trace of detergent kills fish. Keep a dedicated bucket for the tank.
• Adding tap water without conditioner - UK tap water contains chlorine and chloramine, both toxic. Conditioner is not optional if you want to keep fish alive.
• Changing 100% of the water - this destroys the filter bacteria and shocks the fish. Leave the 50% emergency cap alone.
• Dumping cold water straight in - a sudden temperature drop of a few degrees stresses fish and can trigger white spot, so match the temperature when you refill.
• Forgetting the heater - always switch it off before the water level drops, and check it's holding the right temperature when you switch it back on.

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Last reviewed: 1 August 2026 · Evidence-based content · Contains affiliate links