How to clean a fish tank: a safe weekly and monthly routine for beginners

To clean a fish tank safely, change about 20 to 30% of the water each week, vacuum the gravel as you drain, and refill with dechlorinated water at the same temperature. Leave the fish in, and rinse the filter in the old tank water, never under the tap.

The goal isn't a sparkling, sterile tank. It's removing waste while keeping the good bacteria that live in your filter and gravel. Those bacteria turn fish waste into less harmful substances, and scrubbing everything at once is one of the most common ways beginners accidentally kill their fish.

What you need

A gravel vacuum (siphon): a tube with a wide end that sucks up dirt and water together.
One or two buckets used only for the aquarium. Never use a bucket that has had soap or cleaning products in it.
Water conditioner (dechlorinator) that treats both chlorine and chloramine.
A thermometer.
An algae scraper, pad or magnetic cleaner. Use an acrylic-safe one for acrylic tanks.
A water test kit for ammonia, nitrite, nitrate and pH.
An old towel for drips.

How often

When	What to do
Daily	Check the temperature, look at every fish, remove uneaten food and dead leaves
Weekly	Change 20 to 30% of the water, vacuum part of the gravel, wipe the glass, test the water
Every 2 to 4 weeks	Rinse the filter sponge or media in old tank water (when flow drops or it looks clogged)
Monthly	Trim plants, clean the lid and light cover, check the heater and equipment, clean the outside glass
Every few months	Clean filter tubes and impellers; replace worn-out filter media a little at a time

Heavily stocked tanks, goldfish tanks and small tanks (under about 40 litres, or 10 gallons) may need bigger or more frequent changes. The Merck Veterinary Manual suggests up to 50% a week for small bowls and betta tanks. Test nitrate to see how your tank is doing: if it keeps climbing between changes, change more water or feed less.

Step by step: the weekly clean

Wash your hands and arms with plain water and rinse well. Lotions, perfume, soap and hand cream can poison fish (see soap in the tank).
Unplug the heater and filter. A heater that is out of the water while switched on can crack. Leave both off until the tank is refilled.
Clean the inside glass first, so the algae you scrape off gets siphoned out. See "Glass and algae" below.
Vacuum the gravel. Start the siphon, push the wide end into the gravel, and let dirt lift up the tube. Pinch or lift the tube before gravel follows. Work across part of the tank each week (for example, a third or a half), rather than deep-cleaning all of it every time. Around plant roots, just hover over the surface.
Stop at your target. Watch the water level and the fish. Don't remove more than about half the water in one go; big changes can cause sudden temperature and pH swings.
Prepare the new water. Match the temperature within about 1 to 2°C (2 to 4°F). Use your hand only as a rough guide; a thermometer is better.
Add dechlorinator. Follow the label. Most say to dose for the volume of new water, and some recommend dosing for the whole tank when filling straight from a tap or hose. Tap water chlorine and chloramine kill fish and filter bacteria (see chlorine).
Pour gently. Pour onto a plate, a rock or your hand so you don't blast the gravel and stir up dirt.
Plug the heater and filter back in. Prime the filter if it needs it, and check it's running.
Watch the fish for a few minutes. Normal behaviour, maybe some excitement, is a good sign.

Cleaning the filter

The filter is the most important part of your tank. Its sponge and media hold the bacteria that break down fish waste. Treat them gently.

Rinse in old tank water. During a water change, put some of the removed water in a bucket, then squeeze or swish the sponges and media in it until most of the brown sludge is gone. It doesn't need to look new.
Never rinse media under the tap. Chlorine in tap water kills the good bacteria.
Don't replace all the media at once. Ignore packaging that says to replace cartridges every month. Only replace media when it's falling apart, and then change no more than a quarter to a third at a time, a few weeks apart. If your filter only takes cartridges, rinse the old one in tank water and keep using it until it falls apart, or add a small sponge to the filter so some bacteria survive each swap.
Clean tubes and the impeller (the spinning part) every few months with a small brush. This restores flow.
Avoid deep-cleaning the filter and the whole gravel bed on the same day. Spread the work out so the bacteria aren't all disturbed at once.

Glass and algae

Inside: use an algae pad, magnetic cleaner or, on glass tanks only, a razor scraper. Razors and rough pads scratch acrylic.
Outside: use a cloth dampened with plain water or a little white vinegar. Never spray household glass cleaner near the tank; many contain ammonia or other chemicals.
Don't chase every speck of algae. A little algae on decor and the back glass is normal and healthy. Snails and some fish graze on it. For algae problems, see algae, brown algae and black beard algae.
Decor: scrub ornaments and rocks in old tank water with a brush kept just for the tank. Never use soap or detergent.

Common mistakes

Using soap, detergent or household cleaners on anything that goes in the tank, including buckets and sponges.
Washing everything at once: filter, gravel and decor all cleaned on the same day. This can wipe out the bacteria and cause an ammonia spike.
Taking the fish out to clean. Netting and moving fish is more stressful than a careful water change.
Replacing all the filter media or rinsing it under the tap.
Emptying the whole tank and starting again. This usually restarts the nitrogen cycle.
Skipping dechlorinator, even once.
Adding cold or hot water without checking the temperature.
Only topping up for evaporation without ever removing water. Waste and acids build up, and pH can drop over time (often called "old tank syndrome"; see pH crash).
Leaving the heater on while the water is low.

Products that help

Gravel vacuum: removes waste and water in one step. Choose a size that suits your tank; small tanks need a small one.
Hose-style water changer that connects to a tap: saves carrying buckets in large tanks. Remember to dose dechlorinator.
Water conditioner for chlorine and chloramine: makes tap water safe. Use every time.
Liquid test kit: nitrate tells you whether your routine is keeping up.
Thermometer: a stick-on strip or an infrared thermometer for matching temperatures.
Magnetic glass cleaner or algae pad: keeps the view clear without putting your hands in the water.
Spare filter sponge: keep one running in your filter so you always have seeded media for emergencies or a hospital tank.
Brushes and long tweezers: for filter tubes, decor and removing dead leaves.

Why cleaning matters

Fish live in their own waste. The filter's bacteria turn toxic ammonia into nitrite and then nitrate, which is much less harmful, but nitrate and other waste keep building up until you remove them with water changes. Uneaten food and waste in the gravel also feed algae, snails and worms. Regular, gentle cleaning keeps the water stable, which is what fish need most.

When to get help

Ask for help if fish gasp, hide or die after a water change (check temperature, dechlorinator and pH, and read the guide to fish dying), if ammonia or nitrite show up in a tank that's more than two months old, or if the water stays cloudy or smelly despite cleaning. A good local fish store can test your water and look at your routine.

In the game

Multiple Tank Syndrome doesn't have a tank-cleaning card yet. Related cards include "Soapy hands", which is what happens when cleaning goes wrong, "Ammonia spike" and "Better filtration".

Sources

Aquatic Veterinary Services (Dr Jessie Sanders). How to clean a fish tank the right way in 12 easy steps.
Merck Veterinary Manual. Providing a home for fish.
Merck Veterinary Manual. Management of aquarium fish (old tank syndrome).