Start with a partial water change, not a full drain

The most common mistake is emptying the tank completely and scrubbing it clean. That destroys the bacteria colony living 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 reach, and refill with fresh water at the same temperature as what's in the tank.

If your tank is heavily soiled—thick algae coating the glass, visible debris on the bottom, or a foul smell—you may need to remove 40 to 50 percent of the water instead. But never drain it completely unless the tank is empty of fish and you are moving or breaking it down entirely.

Key Takeaways

  • Remove 25 to 30 percent of the water at each cleaning; a full drain kills the beneficial bacteria that keep your tank safe for fish.
  • Use a gravel vacuum to siphon debris from the substrate while you drain water, which removes waste without disturbing the bottom layer where bacteria live.
  • Clean the glass with a soft algae scraper or magnetic cleaner, never with soap or chemicals—only water and a soft cloth or sponge.
  • Rinse the filter media in old tank water you've removed, not in tap water, to preserve the bacteria that live there.
  • Refill with dechlorinated water at the same temperature as the tank to avoid shocking your fish.

Use a gravel vacuum to clean the substrate

A gravel vacuum is a tube attached to a siphon hose that lets you remove water and debris from the bottom of the tank at the same time. Push the tube gently into the substrate—usually gravel or sand—and the suction pulls up waste, uneaten food, and dead plant matter while the heavier substrate falls back down. This is the fastest way to remove the sludge that accumulates on the bottom without stirring up the entire tank.

Start at one corner and work your way across the tank floor. Move slowly so you don't uproot live plants or disturb fish hiding spots. The water that comes out is the water you're removing for your partial change, so you're cleaning and draining at once. Stop when you've removed the amount of water you planned—usually 25 to 30 percent of the tank's total volume.

Clean the glass without chemicals

Algae buildup on the glass is normal and harmless to fish, but it blocks your view. Use a soft algae scraper, a plastic or rubber blade designed for aquarium glass, or a magnetic cleaner that works from both sides of the glass at once. Never use soap, vinegar, bleach, or any chemical cleaner—even small amounts can poison fish and kill beneficial bacteria.

If you use a scraper, hold it at a shallow angle and push gently to avoid scratching acrylic tanks (glass is tougher and harder to scratch). A soft sponge or cloth dampened with water from the tank works for light algae. For stubborn spots, let the scraper sit on the glass for a few seconds to soften the algae, then scrape again.

Rinse the filter media in old tank water

Your filter cartridge or media traps waste and feeds the bacteria that keep the tank safe. If you rinse it under tap water, the chlorine and temperature change kill those bacteria. Instead, rinse the filter in the water you just removed from the tank—the "old" water sitting in your bucket. Squeeze or swish the media gently until the water runs clearer, then put it back in the filter.

If your filter is very clogged and the old tank water doesn't clean it well enough, use a second bucket of old tank water for a second rinse. Only use tap water if the filter is so blocked that water barely flows through it, and even then, let the tank run for a few days before doing another water change to let bacteria recover.

Match the temperature of the new water

Fish are sensitive to sudden temperature swings. Before you refill the tank, check the temperature of the remaining water with a thermometer. Fill a bucket with fresh water and let it sit until it reaches the same temperature, or run it under warm tap water until a thermometer shows it matches. This usually takes 10 to 15 minutes.

If you're adding water that's more than 2 to 3 degrees Fahrenheit different from the tank, the shock can stress fish and trigger disease. Pour the new water slowly into the tank—some people pour it over a plate or into their hand to slow it down and prevent stirring up the substrate again.

Use dechlorinated water if your tap water is treated

Most municipal tap water contains chlorine or chloramine to kill bacteria for human use, but these chemicals harm fish and kill the beneficial bacteria in your tank. If your water comes from a city supply, you have two options: let the water sit in an open bucket for 24 hours (chlorine evaporates, but chloramine does not), or add a dechlorinator product before adding it to the tank.

Dechlorinator comes as a liquid or powder and neutralizes chlorine and chloramine when ready. A small bottle lasts months because you only need a few drops per gallon. If you use well water or know your tap water is not treated, you don't need a dechlorinator—but it doesn't hurt to use one anyway.

How often to wash your aquarium

A lightly stocked tank with good filtration needs a partial water change once a week. A heavily stocked tank or one with live plants may need two changes a week. If you notice algae buildup on the glass or a smell coming from the tank, increase the frequency. If the water stays clear and the tank smells fresh, you may be able to stretch it to every 10 days.

The filter should be rinsed every two to four weeks, depending on how much waste accumulates. If you have a two-stage filter (a mechanical stage and a biological stage), clean only the mechanical stage regularly and rinse the biological stage less often—once a month or less—to keep bacteria alive.

Frequently Asked Questions

Can I use a regular sponge or cloth to clean the glass?

Yes, a soft sponge or microfiber cloth works fine for light algae. Make sure it's never been used with soap or chemicals. For heavier algae, a plastic algae scraper or magnetic cleaner is faster and easier. Never use a rough scouring pad or steel wool—these scratch glass and acrylic.

What if I accidentally drain the tank completely?

Don't panic. Refill it when ready with dechlorinated water at the right temperature. The bacteria in your filter and substrate can survive a few hours without water. Your tank will likely develop a spike in ammonia or nitrite over the next week or two, so do a 25 percent water change every other day for a week to help the bacteria recover. Watch your fish for signs of stress like gasping at the surface or hiding.

Do I need to remove my fish before washing the tank?

No. A partial water change and gentle cleaning while fish are in the tank is normal and safe. If you're doing a deep clean or removing decorations, you can move fish to a temporary container filled with tank water, but for routine maintenance, they can stay put.

Can I use vinegar to clean algae off the glass?

Vinegar is not toxic to fish in small amounts, but it's unnecessary and can lower the pH of your water if some gets into the tank. A plastic scraper or soft cloth does the job just as well with no risk. Stick to water-only cleaning for safety.

How do I know if my filter needs cleaning?

If water flow slows down noticeably or the filter makes a louder noise than usual, the media is clogged. You can also look at the filter cartridge—if it's dark brown or black instead of light tan or white, it's time to rinse it. A clogged filter reduces oxygen in the tank and stresses fish.