How to Change Fish Tank Water: UK Beginner's Guide

Changing your fish tank water is the single most important maintenance task you'll do as an aquarium owner. Get it right and your fish stay healthy, your water stays clear, and algae stays manageable. Get it wrong and you can stress your fish, crash your cycle, or waste hours scrubbing glass that never stays clean.

This guide walks through exactly how often to change your water, what equipment makes the job quick, and the mistakes that trip up most UK beginners. Whether you've got a 20-litre nano tank or a 200-litre community setup, the principles are the same.

How Often Should You Change Fish Tank Water?

The short answer: 25-30% of your tank water every 1-2 weeks. That's the baseline most tropical fish keepers in the UK follow, and it works because it keeps nitrate levels stable while removing dissolved waste that filters can't catch.

But the real answer depends on a few factors:

• Tank size. Larger tanks dilute waste more effectively. A 100-litre tank can often go 2 weeks between changes, while a 30-litre nano might need weekly attention.
• Stocking level. More fish means more waste. A heavily stocked tank needs more frequent changes than a lightly stocked one.
• Filtration. A good canister filter reduces the frequency you need, but doesn't replace water changes entirely. Chemical filtration removes some toxins, but nitrates still build up.
• Live plants. Planted tanks absorb some nitrate, which can stretch your schedule by a few days. Don't rely on plants alone though, even with a dense planted setup.

If you're just starting out, change 25% every 7 days and adjust from there. Test your water weekly with strips like the API 5-in-1 Test Strips to see how your nitrate levels trend between changes.

What Equipment Do You Need?

You don't need much, and most of it costs under £15. Here's the essentials:

Gravel Cleaner or Siphon

This is the most important tool. A gravel cleaner lets you siphon water out while simultaneously vacuuming debris from the substrate. The Luigi's Syphon Fish Tank Cleaner is a popular 3-in-1 option that handles water changes, gravel cleaning, and has a built-in filter to catch debris. For most UK tanks up to 150 litres, it does the job well.

If you've got a larger tank, a dedicated gravel vacuum with a wider tube picks up debris faster. The key thing is getting something with a siphon mechanism, not just a turkey baster approach that stirs up more than it removes.

Water Conditioner

UK tap water contains chlorine and chloramine, both of which kill beneficial bacteria and irritate fish gills. You must treat new water before it goes into the tank. Two reliable options available on Amazon UK:

• AQUARIAN Tap Water Safe - a concentrated formula, a few drops per litre treats a standard tank
• AquaCare Tap Water Conditioner - 240ml bottle that covers a lot of water changes for the price

Both work. The main difference is concentration and bottle size. If you're doing weekly changes, the larger bottle works out cheaper per change.

Water Test Kit

Before you change water, test what you're removing. API 5-in-1 Test Strips check pH, nitrite, nitrate, carbonate hardness, and general hardness in one dip. They're not as precise as liquid test kits, but for routine monitoring they're fast and accurate enough to spot problems early.

A Bucket

Use a dedicated bucket that's never held soap, bleach, or cleaning products. A standard 10-litre bucket works for most tank sizes. Some aquarists keep two, one for siphoning out and one for pre-mixed replacement water.

Step-by-Step: How to Change Your Fish Tank Water

Step 1: Prepare the New Water

Fill your bucket with cold tap water and add the recommended dose of water conditioner. Let it sit for a few minutes while you prepare the tank. Some keepers prefer to match the temperature by floating the bucket near the tank or using a thermometer, but most UK tropical setups are stable enough that a few degrees difference won't matter if the new water is room temperature.

Step 2: Clean the Glass

Before siphoning, wipe down the inside of the glass with an algae pad. This knocks loose any algae that's built up, and the debris gets siphoned out with the old water. Clean the outside glass too, since you're already there.

Step 3: Start the Siphon

Insert your gravel cleaner into the substrate and start the siphon. Move it slowly across the gravel, pausing over areas where waste collects. You'll see debris get sucked up through the tube. Don't dig too deep, especially with fine substrates like sand, or you'll disturb the bacterial colonies living in the top layer.

Siphon until you've removed roughly 25-30% of the tank volume. For a 100-litre tank, that's about 25-30 litres, or 2.5-3 bucketfuls.

Step 4: Add the New Water

Pour the conditioned replacement water into the tank slowly. Pouring it directly onto the substrate stirs up debris, so aim for a gentle flow. Some people pour over a clean plate or their hand to break the flow. Top up to your normal water level.

Step 5: Check the Parameters

Test the water after the change. Your nitrate should be lower, pH should be stable, and ammonia and nitrite should read zero. If ammonia or nitrite spike after a water change, something's wrong with your cycle, not the water change itself.

Water Conditioning: Why It Matters

UK tap water is treated with chlorine or chloramine to make it safe for drinking. Both are toxic to fish and will kill the beneficial bacteria in your filter. Never add untreated tap water to an aquarium.

A product like API Stress Coat goes a step further by also replacing the fish's protective slime coat, which gets damaged during handling or tank maintenance. It's not essential for every water change, but useful if you've been doing maintenance that disturbs the fish, or if you're adding new fish to the tank.

Some UK areas have very hard water. If your GH reading is consistently high, you might want to mix tap water with RO (reverse osmosis) water for sensitive species. But for most community fish, standard treated tap water works fine.

Common Mistakes to Avoid

Changing too much water at once. Replacing more than 50% of your water in one go can shock fish because the parameters shift too quickly. Stick to 25-30% unless you're dealing with an emergency like an ammonia spike.

Forgetting the water conditioner. Even a small amount of chlorinated water damages your filter bacteria. Always treat, even if you're only topping up a few litres.

Cleaning the filter at the same time. Don't deep-clean your filter media during a water change. If you replace or thoroughly wash all the media at once, you remove the beneficial bacteria that process waste. If the filter needs cleaning, rinse the media in old tank water, not tap water, and only clean one section at a time.

Siphoning too aggressively. Sticking the gravel cleaner too deep into the substrate disturbs the bacterial colonies. Gentle, slow passes pick up debris without disrupting the substrate ecosystem.

Ignoring water temperature. In winter, your replacement water might be significantly colder than the tank. A sudden temperature drop stresses fish. Let the replacement water reach room temperature, or use a digital thermometer to check it's within 2 degrees of the tank.

How to Handle Specific Situations

New tank setup: During the first 4-6 weeks, your tank is cycling. Water changes during this period are smaller, around 10-15%, and focus on keeping ammonia and nitrite low enough for your fish to survive. See our guide on adding bacteria to a new fish tank for more detail.

After adding new fish: Do a 20% water change the day before and the day after adding new fish. This helps dilute any ammonia spike from the new bioload.

High nitrate readings: If your nitrate is consistently above 40ppm between changes, increase your water change percentage or frequency. A 30% weekly change usually brings levels under control for most setups.

Algae problems: Excess nitrate fuels algae growth. Regular water changes are one of the most effective algae controls. Combine with reduced lighting hours and you'll see a noticeable difference within two weeks.

FAQs

Can I use bottled water for my fish tank?

You can, but there's no benefit. Bottled water is either just filtered tap water or mineral water with inconsistent mineral content. Treated tap water is more reliable and far cheaper. If your tap water is heavily chlorinated, a good conditioner handles it.

Should I change all the water if there's an ammonia spike?

No. A large water change during an ammonia spike can shock fish. Do a 25% change, add conditioner, and test again in a few hours. If ammonia persists, add a filter bacteria supplement and do another small change the next day.

How do I know if my water change is working?

Test your water before and after. Your nitrate should drop, ammonia and nitrite should stay at zero, and pH should be stable. If parameters aren't improving, you may need to increase the percentage or frequency of changes.

Do I need to change water in a planted tank?

Yes, but less frequently. Live plants absorb some nitrate, which can extend your schedule to every 2-3 weeks in a well-planted setup. Still test regularly to make sure nitrate doesn't creep up.

Is it normal for fish to hide after a water change?

Yes, temporarily. Fish often hide for a few hours after a water change because the parameters shift slightly. If they're still hiding after 24 hours, check your water parameters for problems.

Can I do water changes too often?

Technically yes, but it's rare in practice. Changing small amounts daily isn't harmful, it's just unnecessary work. Over-frequent large changes can stress fish by constantly shifting parameters. Stick to the 25-30% weekly or fortnightly routine and you'll be fine.

M

Mark Whitfield

Mark writes about pet products and habitats for WhoPets. He has kept freshwater aquariums for over 8 years and compares pet gear using Amazon purchaser reviews, manufacturer specs and independent feedback.

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Last reviewed: August 28, 2026 · Evidence-based content · Contains affiliate links