Start with a partial water change, not a full drain

The most common mistake is emptying the tank completely and scrubbing it clean. That kills the bacteria colony in your filter and substrate that keeps ammonia and nitrite from poisoning your fish. Instead, remove about 25 to 30 percent of the water, clean what you can see, and refill with fresh water at the same temperature.

A partial water change every week or two is the backbone of aquarium maintenance. It removes dissolved waste that your filter cannot catch, lowers nitrate buildup, and keeps the water chemistry stable enough that your fish stay healthy. The bacteria that do the real work of breaking down fish waste live in your filter media and on surfaces inside the tank—leaving them alone is what keeps the system running.

Key Takeaways

  • Remove 25 to 30 percent of the water weekly or biweekly; never drain and refill the entire tank unless the glass is cracked or the fish are sick.
  • Use a gravel vacuum to siphon debris from the substrate while you drain, which removes waste without disturbing the bacteria that filter your water.
  • Rinse filter media in old tank water, not tap water, so you do not kill the bacteria that live on it.
  • Clean the glass, decorations, and intake tubes with a soft brush or algae scraper, but leave the filter and substrate mostly undisturbed.
  • Match the temperature of fresh water to the tank before adding it back, because a sudden change stresses fish and can cause disease.

Use a gravel vacuum to clean the substrate in one step

A gravel vacuum (also called a siphon or aquarium vacuum) is a tube with a wider chamber at the top. You push the narrow end into the gravel, and water and debris flow up into the chamber and out through a hose into a bucket. As you work, the gravel settles back down, but the waste and uneaten food get pulled out.

Start at one corner of the tank and work slowly across the bottom. Push the tube down until it touches the glass, then pull it back up slightly—you want to disturb the gravel enough to loosen trapped waste, but not so much that you uproot plants or bury decorations. Spend extra time near the filter intake and in corners where debris collects. Stop when you have removed the amount of water you planned to drain (usually 25 to 30 percent).

If you do not have a gravel vacuum, a small net or turkey baster can remove visible waste from the surface, though it is slower and less thorough. For a bare-bottom tank (no substrate), straightforward siphon water into a bucket using a length of airline tubing.

Rinse the filter media in old tank water

The filter cartridge or media is where most of your beneficial bacteria live. Rinsing it under tap water kills those bacteria and forces the tank to cycle all over again—which means ammonia and nitrite spike, and fish get sick or die. Instead, rinse the media in the old tank water you just removed.

Squeeze or swish the filter media gently in a bucket of the old water until it looks cleaner and the water in the bucket turns cloudy. You are removing trapped debris and dead algae, not sterilizing the media. If the media is falling apart or has a strong smell, it is time to replace it—but even then, keep the old media in the tank for a week or two while the new media builds up its bacteria colony.

Some filters have multiple stages: a coarse sponge, a finer sponge, and activated carbon. Clean them in order from coarsest to finest, and replace the carbon every month or two (or when it stops removing odor). The sponges can last for years if you rinse them gently.

Clean the glass, decorations, and intake tubes

While the water level is down, wipe the inside of the glass with a soft cloth or a dedicated algae scraper. Plastic scrapers work on acrylic tanks; glass tanks can handle metal blades, but plastic is safer if you are not sure. Scrape gently in one direction to avoid scratching. For stubborn algae, let the scraper sit on the spot for a few seconds to soften it first.

Decorations, plants, and the filter intake tube collect algae and debris too. Use a soft brush (an old toothbrush works) to scrub them while they are still in the tank, or remove them and scrub them in a separate bucket of old tank water. Do not use soap or chemicals—even a tiny residue can poison fish. If something is heavily coated in algae, soak it in old tank water for a few minutes to loosen it.

The intake tube of your filter is a common clog point. If water flow has slowed, disconnect the tube and rinse it out or use a small brush to clear it. A clogged intake forces the filter to work harder and can cause it to overheat.

Refill with fresh water at the right temperature

Before you add water back, check the temperature of the fresh water with a thermometer. It should match the tank temperature within a degree or two. Cold water shocks fish and can trigger disease; hot water can cook them. If the fresh water is too cold, let it sit in a bucket for a while, or mix in some hot water from the tap (test it first).

Pour the fresh water slowly into the tank, ideally into a bucket or over a decoration so it does not blast the substrate and cloud the water. If you have plants, pour away from them to avoid uprooting them. Top up until the water level is back where it started—usually marked by a line on the glass or the height of the filter intake.

Turn the filter back on if you turned it off, and watch for a few minutes to make sure water is flowing normally and no leaks have appeared. The tank should clear within an hour; if it stays cloudy, the filter may be clogged or the substrate may have been disturbed too much.

Deep clean the tank only when necessary

A full tank cleaning—draining everything, scrubbing the glass and substrate, and refilling—should happen only if the tank has a strong smell, the glass is heavily coated, or you are treating a disease. Even then, try to save at least some of the old water and old filter media to preserve your bacteria colony.

If you must do a full clean, set up a temporary home for your fish first: a bucket, a spare tank, or even a large plastic storage container with an air stone and heater. Drain the tank completely, scrub the glass and any decorations with a soft brush and old tank water (no soap), and rinse the substrate by stirring it in a bucket of water until the water runs clear. Refill the tank, let it run for an hour to settle, then move the fish back in. This process stresses fish and can cause them to get sick, so avoid it unless the tank is genuinely dirty.

Set a cleaning schedule and stick to it

Weekly or biweekly partial water changes prevent the buildup that leads to big cleaning jobs later. Mark a day on your calendar—Sunday morning, for example—and spend 15 to 20 minutes on it. Drain, vacuum, rinse the filter, refill, and you are done.

In between water changes, watch for algae on the glass and remove it with a scraper when it gets thick. If you see uneaten food on the bottom, remove it with a net. A few minutes of maintenance every few days keeps the tank from becoming a chore.

The frequency depends on tank size, how many fish you have, and how much you feed them. A small tank with many fish needs water changes every week; a large tank with few fish might be fine every two weeks. If nitrate levels stay below 40 parts per million (ppm) on a test kit, your schedule is working. If they creep higher, increase the frequency or the amount of water you change.

Frequently Asked Questions

Can I use tap water straight from the faucet?

Tap water is usually safe for fish, but it often contains chlorine or chloramine, which kill beneficial bacteria and can irritate fish gills. Let tap water sit in an open bucket for 24 hours so chlorine evaporates, or use a dechlorinator product (a few drops per gallon). If your tap water is very hard or very soft, a water conditioner can help balance it.

What if my filter stops working during a water change?

Turn it back on as soon as you refill the tank. A filter that runs dry can overheat or suck air into the lines. If it does not restart, check for clogs in the intake tube or media, and make sure the power cord is plugged in. If it still will not work, replace it before the tank cycles and ammonia spikes.

How do I know if I am cleaning too much or not enough?

Test the water with a kit that measures ammonia, nitrite, and nitrate. Ammonia and nitrite should always be zero; nitrate should stay below 40 ppm. If nitrate is creeping up, increase water change frequency. If ammonia or nitrite appears, the tank is not cycling properly—usually because the filter media was rinsed in tap water or the tank is overstocked.

Can I clean the filter and do a water change on the same day?

Yes, but do the water change first, then rinse the filter in the old water you removed. If you rinse the filter and then drain the tank, you lose the old water and have to use tap water for the rinse, which kills bacteria.

What should I do if the tank smells bad?

A strong smell usually means waste is building up faster than the filter can handle. Increase water change frequency to every three or four days for a week or two, and check that you are not overfeeding. If the smell persists, the filter media may be clogged or the tank may be overstocked. A partial water change and a filter cleaning usually fix it.