How to Clean a Fish Tank Without Killing the Bacteria

The biggest mistake in cleaning a fish tank is cleaning it too well. It feels right to strip everything down, scrub the gravel and rinse the filter under the tap, but that wipes out the beneficial bacteria that keep your fish alive. A good clean is gentle and partial, not a full reset. Done properly, a weekly ten-minute routine keeps the water healthy without ever crashing the tank.

This guide walks through a safe cleaning routine, how much water to actually change, and the two things you should never do. It assumes your tank is already cycled; if it is not, start with our guide on how to cycle a fish tank.

The quick routine

A healthy maintenance clean is mostly a partial water change plus a light tidy:

1. Change 20 to 30 percent of the water each week.
2. Vacuum the gravel where you change the water, to lift out waste.
3. Wipe the glass and scrape any algae before it builds up.
4. Rinse filter media in old tank water only, never under the tap.

The amount of water matters, and it is based on your tank’s real volume. A 20 percent change on a 120 litre tank is 24 litres. Work out your true water volume with the water volume calculator, since substrate and decor mean the tank holds less than its rated size, then take a fifth to a third of that each week.

How much water should you change?

Water changes dilute nitrate and replace trace minerals. For most tanks, 20 to 30 percent weekly is the sweet spot: enough to keep nitrate low, not so much that you shock the fish with a big swing in parameters.

Stocking level	Change	Frequency
Lightly stocked / planted	15 to 20%	Weekly
Standard community	20 to 30%	Weekly
Heavily stocked / messy	30 to 40%	Weekly, or twice weekly

Always match the new water’s temperature to the tank and treat it with dechlorinator before it goes in. Chlorine and chloramine from tap water harm both fish and the filter bacteria.

The two things you must never do

• Never rinse filter media under tap water. Chlorinated water kills the bacteria living in the filter, which is your entire biological filtration. Swish the media in a bucket of the water you just removed instead.
• Never deep-clean the whole tank at once. Replacing all the water, scrubbing the gravel and cleaning the filter in one go removes too much bacteria and can trigger a mini-cycle that spikes ammonia. Spread jobs out across weeks.

These two mistakes cause more “my water went cloudy and my fish got sick after I cleaned” problems than anything else.

Vacuuming the gravel

A gravel vacuum, or siphon, does two jobs at once: it removes the water for your water change and lifts detritus out of the substrate. Push it gently into the gravel, let the debris rise and get sucked away, then move to the next patch. You do not need to vacuum the whole bed every week; cover a different third each time so you never disturb too much of the biological activity at once.

In a planted tank, vacuum lightly over the substrate rather than digging into it, to avoid uprooting plants and releasing trapped gas.

Algae and glass

A little algae is normal. Wipe the glass with an algae scraper or magnet before it thickens, and clean the front panel every water change so the view stays clear. If algae is taking over, the usual cause is too much light for the nutrients available, which we cover in how much light a planted tank needs.

Frequently asked questions

How often should I clean my fish tank? A partial water change and light clean once a week suits most tanks. Heavily stocked tanks may need it twice a week; lightly stocked planted tanks can sometimes stretch to every ten days.

How much water should I change in a fish tank? Usually 20 to 30 percent of the water volume each week. Base it on your real water volume, which you can find with the water volume calculator.

Can I use tap water to clean my fish tank? Only if you treat it with dechlorinator first and match the temperature. Never rinse filter media under the tap, as chlorine kills the beneficial bacteria.

Why did my tank get cloudy after cleaning? Usually because too much was cleaned at once, removing bacteria and triggering a bacterial bloom or mini-cycle. Clean less at a time and never rinse filter media in tap water.

Should I remove the fish to clean the tank? No. Removing fish is stressful and unnecessary for a routine clean. Leave them in and do a gentle partial water change around them.

The bottom line

Cleaning a fish tank well means doing less than you think. Change 20 to 30 percent of the water weekly based on your real volume, vacuum a section of gravel, wipe the glass, and rinse filter media only in old tank water. Find your true water volume with the fish tank volume calculator and water volume calculator, keep the routine consistent, and your tank stays clear and healthy without the crashes that come from over-cleaning.