Clean your aquarium on a weekly schedule, with a full water change every two to four weeks

Most aquariums need a partial water change and filter cleaning once a week. A partial water change means removing about 25 percent of the water and replacing it with fresh, treated water—not draining the tank completely. Weekly maintenance prevents ammonia and nitrite from building up to levels that stress or kill fish.

A full water change—draining and refilling the entire tank—happens less often, usually every two to four weeks, depending on tank size and how many fish you have. Smaller tanks (under 20 gallons) need full changes more often because waste accumulates faster in less water. Larger tanks can go longer between full changes because the water volume dilutes waste more slowly.

The exact schedule also depends on whether your tank has a working filter and how densely stocked it is. A 10-gallon tank with five fish needs more frequent cleaning than a 40-gallon tank with the same number of fish. If your filter is broken or you have no filter, you will need to do partial water changes two or three times a week instead of once.

Key Takeaways

  • Do a partial water change (25 percent) and clean the filter once a week in most tanks.
  • A full water change happens every two to four weeks, with smaller tanks needing it more often.
  • Tanks without filters or with heavy fish loads need water changes two or three times weekly.
  • Clean the filter in old tank water, not tap water, to preserve the bacteria that break down fish waste.
  • Test ammonia and nitrite levels if your fish show signs of stress—gasping at the surface, hiding, or refusing food—as this may mean you need to clean more often.

What happens between cleanings

Fish produce ammonia through their gills and waste. Beneficial bacteria in your filter convert ammonia to nitrite, then to nitrate. Nitrate is less toxic but still builds up over time. If you do not remove water regularly, ammonia and nitrite climb to levels that damage fish gills and weaken their immune system, making them sick or killing them.

The speed at which these toxins build up depends on tank size, number of fish, and filter strength. A small tank with many fish can become toxic in days. A large tank with few fish and a strong filter can stay safe for two weeks or longer between changes. This is why there is no single answer that works for every tank—you have to match the schedule to your specific setup.

How to do a weekly partial water change

A partial water change takes about 15 to 20 minutes. Use a siphon or aquarium gravel vacuum to remove water from the bottom of the tank, where waste settles. Remove about 25 percent of the total water volume. If your tank holds 20 gallons, remove 5 gallons.

While the water is draining, clean the filter. Most filters have a cartridge or foam pad that traps debris. Rinse it in a bucket of old tank water—never tap water. Tap water kills the beneficial bacteria living in the filter, and those bacteria are what keep ammonia from poisoning your fish. If you rinse the filter in tap water, you have to wait weeks for the bacteria to grow back, and your tank becomes toxic in the meantime.

After cleaning the filter, refill the tank with fresh water that has been treated with a dechlorinator. Tap water contains chlorine to kill bacteria for human use, but it also kills the bacteria in your tank. Dechlorinator removes chlorine when ready. Most come as a liquid you add to the bucket before pouring it in. Let the water reach room temperature before adding it, or the temperature shock can stress your fish.

When to do a full water change

A full water change means draining the tank completely and refilling it. Do this every two to four weeks, depending on tank size. Smaller tanks need it more often because waste builds up faster in less water. A 10-gallon tank might need a full change every two weeks; a 55-gallon tank might go four weeks.

Before you drain the tank, move your fish to a temporary container filled with water from the tank—use a net or small cup, not your hands. This keeps them safe while you work and prevents them from being stressed by the draining process. Clean the gravel, decorations, and glass while the tank is empty. Scrub algae off the glass with an algae scraper or old credit card. Rinse the gravel in tap water to remove settled waste.

Refill the tank with treated, room-temperature water and let it run for a few minutes before returning the fish. This gives the filter time to start circulating and the water to reach the right temperature.

Signs your tank needs cleaning more often

If your fish are gasping at the surface, hiding constantly, or refusing food, the water quality has likely dropped. These are signs of ammonia or nitrite stress. Test the water with an aquarium test kit—these measure ammonia, nitrite, and nitrate levels. If ammonia or nitrite are above zero, your tank is toxic and needs an when ready partial water change of 50 percent or more.

Cloudy water, a foul smell, or visible algae blooms also signal that cleaning is overdue. Cloudy water usually means bacteria are multiplying because waste is accumulating. A foul smell means anaerobic bacteria (bacteria that live without oxygen) are breaking down waste in the gravel, which produces hydrogen sulfide. Both require when ready action: do a 50 percent water change and clean the gravel thoroughly.

If you see these signs regularly, your current cleaning schedule is not matching your tank's needs. Increase the frequency of partial changes to twice a week, or reduce the number of fish. A tank that is overstocked will never stay clean enough, no matter how often you change the water.

Filters and cleaning frequency

A working filter reduces how often you need to change water because it removes solid waste and grows bacteria that break down ammonia. A tank with a good filter might need only one partial change a week. A tank with no filter or a weak filter needs changes two or three times a week.

Clean the filter during every partial water change, but do not replace the filter cartridge every time. Most cartridges last four to six weeks before they need replacing. When you replace it, keep the old cartridge in the tank for a few days if possible, so the bacteria can transfer to the new one. If you throw away the old cartridge and install a new one without overlap, you lose most of your beneficial bacteria and the tank becomes toxic for one to two weeks while the bacteria regrow.

Seasonal changes and vacation planning

Cleaning frequency does not change much with seasons in an indoor tank, but water temperature does affect how quickly ammonia builds up. Warmer water speeds up fish metabolism and bacterial growth, so a tank in a warm room may need cleaning slightly more often than one in a cool room. If your tank temperature swings more than a few degrees, use a heater to keep it stable.

If you are going on vacation for a week or more, do a full water change and clean the filter thoroughly before you leave. This gives you the longest possible window before the water becomes toxic. Do not overfeed your fish before you leave—uneaten food rots and fouls the water. Feed them lightly for the last few days, or use an automatic feeder set to dispense small amounts once a day. Most fish can survive two weeks without food, but they cannot survive toxic water for more than a few days.

Frequently Asked Questions

Can I use tap water straight from the faucet?

No. Tap water contains chlorine that kills the beneficial bacteria in your filter and tank. Always treat tap water with a dechlorinator before adding it. Dechlorinator is inexpensive and available at any aquarium store. It works when ready, so you can add treated water to the tank right away.

What if I miss a week of cleaning?

If you miss one week, do a 50 percent partial water change as soon as you remember, then get back on schedule. If you miss two weeks or more, do a full water change and clean the gravel thoroughly. Test the water with a kit to check ammonia and nitrite levels. If either is above zero, do another 50 percent change the next day.

Do I need to remove my fish when I clean the tank?

For a weekly partial water change, no—the fish can stay in the tank. For a full water change, move them to a temporary container filled with old tank water so they are not stressed by the draining and refilling. Never use a net to catch fish unless necessary; use a cup or small container instead to minimize injury.

How do I know if my filter is working?

A working filter pulls water through its cartridge and returns it to the tank. You should see water flowing out of the filter outlet. If the flow is very slow or stopped, the cartridge is clogged and needs cleaning. If the filter is not running at all, check that it is plugged in and the power switch is on. If it still will not run, it may be broken and needs replacement.

What is the difference between ammonia, nitrite, and nitrate?

Ammonia is produced by fish waste and is highly toxic. Beneficial bacteria convert it to nitrite, which is also toxic. Other bacteria convert nitrite to nitrate, which is much less toxic but still builds up over time. Water changes remove all three, but nitrate is the main reason you need to change water regularly—it does not break down further and accumulates until you remove it.