The Basic Steps for Cleaning Aquarium Water

Cleaning aquarium water means removing waste and debris without harming the fish or the beneficial bacteria that keep the tank healthy. The most common method is a partial water change—removing 20 to 30 percent of the water each week and replacing it with fresh water of the same temperature. You do this with a siphon or gravel vacuum, which pulls water out while also cleaning the substrate (the gravel or sand on the bottom). This single step handles most of the work: it removes fish waste, uneaten food, and other dissolved compounds that make water cloudy or toxic.

The second part of cleaning is maintaining the filter, which traps physical debris and houses bacteria that break down ammonia and nitrite. A clogged filter slows water flow and reduces its cleaning power. You rinse the filter media (the sponge, cartridge, or floss inside) in old aquarium water—never tap water, which kills the bacteria you need. Do this every two to four weeks, depending on how dirty it gets.

These two tasks—weekly water changes and monthly filter maintenance—prevent most water quality problems. If you skip them, ammonia and nitrite build up, the water becomes cloudy, and fish become stressed or sick.

Key Takeaways

  • A partial water change of 20 to 30 percent each week removes waste and keeps water parameters stable without disrupting the tank's ecosystem.
  • Use a siphon or gravel vacuum to remove water from the bottom of the tank, which also cleans debris from the substrate.
  • Rinse your filter media in old aquarium water every two to four weeks to keep water flowing without killing beneficial bacteria.
  • Always match the temperature of fresh water to the tank water before adding it, and let tap water sit 24 hours or use a dechlorinator if your tap water is heavily treated.
  • Never use soap, bleach, or harsh chemicals on any part of your tank—these kill fish and bacteria when ready.

Setting Up a Siphon or Gravel Vacuum

A siphon is the simplest tool: a tube that uses gravity to pull water out of the tank. A gravel vacuum is a siphon with a wide tube on the end that stirs up the substrate as you work, pulling out debris trapped in the gravel. Both work the same way. Fill the tube with water (some have a lever or button to start the flow), place one end in the tank and the other in a bucket on the floor below the tank, and water flows out as long as the bucket stays lower than the water level.

Start at one corner of the tank and slowly move the vacuum across the bottom, stirring gently into the gravel. You will see debris—brown or black particles—get sucked up. Avoid the areas where plants are rooted or where fish hide, so you do not uproot plants or stress the fish. Work for about 5 to 10 minutes, depending on tank size. A 20-gallon tank usually takes 10 to 15 minutes total.

Stop when you have removed roughly 20 to 30 percent of the water. Mark the water level on the outside of the tank with a piece of tape so you know where to refill to each week. This consistency matters: removing too much water stresses fish and bacteria, and removing too little leaves waste behind.

Preparing and Adding Fresh Water

The water you add back must match the tank's temperature within 2 to 3 degrees Fahrenheit. Cold water shocks fish and can kill them. Fill a bucket with tap water and let it sit for 24 hours, or use a dechlorinator product (a few drops in the bucket) if your tap water is heavily chlorinated. Some aquarists use a thermometer to heat the water to match the tank before pouring it in slowly.

Pour the fresh water in slowly—over a minute or two—so it mixes gradually with the tank water. Pouring it fast can stir up the substrate and cloud the water, and it can also create a temperature shock. If you have plants or decorations, pour the water onto them rather than directly onto the substrate to slow the flow.

If your tap water is very hard (high mineral content) or very soft, or if you keep fish that need specific water conditions, you may need to adjust the water before adding it. This is beyond basic cleaning, but it is worth researching for your specific fish species.

Cleaning and Rinsing the Filter

The filter is where most of the biological work happens, so cleaning it correctly is critical. Remove the filter media—the sponge, cartridge, floss, or other material inside the filter—and rinse it in a bucket of old aquarium water that you removed during the water change. Squeeze and swish the media gently in the old water until it looks cleaner. The old water contains beneficial bacteria, so rinsing in it preserves those bacteria while removing trapped debris.

Never rinse filter media under tap water. Chlorine and temperature shock kill the bacteria colony you have spent weeks building. If the media is very clogged and old water alone does not clean it well enough, rinse it briefly under cool tap water as a last resort, but understand that you are setting back the bacteria cycle by a few days.

Check the filter housing (the container that holds the media) for algae or slime buildup on the walls. Wipe it with a soft cloth or sponge rinsed in old tank water. Reassemble the filter, make sure it is sealed properly, and turn it back on. The filter should run continuously except during maintenance.

Dealing with Cloudy or Discolored Water

Cloudy water usually means one of three things: a bacterial bloom (the water is milky white), suspended particles (the water looks dusty), or algae (the water is green). A bacterial bloom happens when ammonia spikes, usually because of overfeeding or a missed water change. It clears on its own in a few days once you do a large water change and reduce feeding, but it is a sign that your routine needs adjustment.

Suspended particles mean the filter is not catching debris fast enough. This happens if the filter is clogged, the flow rate is too low, or you stirred up too much substrate during cleaning. Do a water change, clean the filter, and be gentler with the gravel vacuum next time.

Green water is algae, usually caused by too much light or excess nutrients from overfeeding. Reduce light to 8 to 10 hours per day, do a water change, clean the filter, and feed less. Algae will fade over a week or two. Do not use algae-killing chemicals in a tank with fish—they can harm or kill fish and bacteria.

How Often to Clean and What to Avoid

A standard routine is a 20 to 30 percent water change once per week and filter cleaning every two to four weeks. Tanks with more fish, more plants, or more food waste need more frequent changes—sometimes twice per week. Tanks with fewer fish and plants can sometimes go longer, but weekly is the safest baseline.

Never clean the entire tank at once. Draining it completely, scrubbing everything, and refilling it kills the bacteria colony and causes an ammonia spike that can kill fish. If the tank is very dirty or has a bad smell, do two large water changes a few days apart instead of one massive cleaning.

Avoid soap, bleach, vinegar, and other household cleaners on any part of the tank or equipment. These are toxic to fish and bacteria. If you need to remove algae from the glass, use a plastic scraper or algae pad (not metal, which scratches glass). For decorations or equipment, rinse them in old tank water or plain tap water only.

Testing Water Quality

A water test kit tells you whether your cleaning routine is working. Basic kits measure ammonia, nitrite, and nitrate—the three compounds that build up in a tank. Ammonia and nitrite should always be zero. Nitrate will be higher (usually 20 to 40 parts per million) and is controlled by water changes. If ammonia or nitrite are above zero, your filter is not mature yet, you are overfeeding, or you are not doing water changes often enough.

Test strips are faster but less accurate than liquid kits. A liquid test kit costs more upfront but lasts longer and gives more reliable readings. Test once per week when the tank is new (the first 4 to 6 weeks), then once per month after that. If you see ammonia or nitrite spike, increase water change frequency when ready and reduce feeding.

pH and hardness are also measurable, but they matter less for most community fish. Focus on ammonia, nitrite, and nitrate first. Once those are stable, you can explore other parameters if your fish species requires it.

Frequently Asked Questions

Can I use tap water straight from the faucet?

Tap water is usually safe for fish, but it often contains chlorine, which kills fish and bacteria. Let tap water sit for 24 hours to let chlorine evaporate, or use a dechlorinator product (a few drops per gallon). Temperature matters too—match the fresh water to the tank temperature before adding it.

What if I miss a week of water changes?

One missed week usually does not harm fish, but ammonia and nitrite will start to rise. Do the water change as soon as you remember, and reduce feeding for a few days to let the tank recover. If you miss multiple weeks, do two large water changes a few days apart instead of one huge change, which can shock the fish.

How do I know if my filter is clogged?

Water flow slows noticeably, or the filter makes a grinding sound. The media will look dark brown or black instead of light tan or white. Clean the filter media in old tank water. If flow is still slow after cleaning, the media may be worn out and need replacement.

Is it bad to do water changes too often?

Changing more than 50 percent of the water per week can stress fish and remove too many nutrients that plants need. Stick to 20 to 30 percent weekly. If the tank is very dirty, do two 25 percent changes a few days apart rather than one 50 percent change.

What should I do if the water smells bad?

A foul smell means ammonia or hydrogen sulfide is building up, usually from overfeeding or a dead fish. Check for dead fish or uneaten food trapped in the substrate. Do a large water change (30 to 40 percent), clean the filter, and reduce feeding. If the smell returns, test the water for ammonia and nitrite.