Clean your aquarium every one to two weeks, with a partial water change of 25 to 50 percent

The standard schedule for most home aquariums is a partial water change every 7 to 14 days. You remove roughly a quarter to half of the water, vacuum the gravel or substrate to pull out waste, and wipe down the glass. This routine removes the ammonia and nitrate that build up from fish waste and uneaten food — the main reason tanks get cloudy, smell, or harm the fish living in them.

How often you actually need to clean depends on three things: how many fish you have, how much you feed them, and the size of your tank. A 10-gallon tank with five fish needs cleaning every 7 days. A 55-gallon tank with the same five fish can go 10 to 14 days. An overstocked tank — one where you crammed in too many fish — may need cleaning twice a week. The rule is straightforward: more fish and less water means more frequent cleaning.

Never do a full water change unless the tank is cloudy, the water smells, or you are treating a disease. Removing all the water kills the beneficial bacteria that live in the gravel and filter, and those bacteria are what keep ammonia from poisoning your fish. A partial change is safer and works better.

Key Takeaways

  • A partial water change of 25 to 50 percent every 7 to 14 days is the standard maintenance schedule for most home aquariums.
  • Tanks with more fish or smaller volumes need cleaning more often — sometimes twice a week — because waste builds up faster.
  • Vacuum the gravel during each water change to remove settled waste that bacteria cannot break down fast enough.
  • Never drain the entire tank during routine cleaning, because you will remove the beneficial bacteria that filter ammonia and keep fish alive.
  • A filter that is clogged with debris will slow water flow and reduce the tank's ability to handle waste, so rinse the filter media every two to four weeks.

What happens if you clean too often or not often enough

Cleaning too often — more than twice a week on a normal tank — removes the bacteria that break down fish waste. These bacteria live in the gravel, on the glass, and inside the filter. When you remove too much water or replace the filter too often, you reset the bacterial colony and ammonia spikes. New tanks (less than four weeks old) are especially vulnerable because the bacteria colony is still small. If you change the water more than once a week in a new tank, ammonia will climb and fish will die.

Not cleaning often enough lets ammonia and nitrate build up to levels that stress or kill fish. The water turns cloudy, smells like rotten eggs, and the fish stop eating or gasp at the surface. Algae blooms because excess nutrients feed it. Once the tank reaches this state, a single large water change will not fix it — you have to do partial changes every few days for a week or two to bring the chemistry back down safely.

How to do a partial water change the right way

Turn off the heater and filter before you start. Use a gravel vacuum — a tube with a bulb or electric pump that siphons water while pulling debris from the bottom. Push the tube into the gravel in several spots around the tank, moving slowly so the vacuum pulls out settled waste but not the gravel itself. This step removes the organic matter that bacteria cannot process fast enough.

Remove the water into a bucket. For a 25 percent change on a 40-gallon tank, you remove 10 gallons. For a 50 percent change, you remove 20 gallons. Do not guess — measure with a bucket or mark the tank's water level before you start so you know how much to remove.

Refill with fresh water that is the same temperature as the tank water. Cold water shocks the fish and stresses them. Let tap water sit in a bucket for 24 hours before adding it, or use a water conditioner to remove chlorine when ready. Turn the heater and filter back on once the tank is full.

Cleaning the filter and glass between water changes

Rinse the filter media — the foam, cartridge, or floss inside the filter — every two to four weeks by running it under tap water in a bucket. Do not use hot water or soap. You are rinsing away the trapped debris, not sterilizing the media. The beneficial bacteria live on the media, and you want to keep them. If the filter is clogged, water flow slows and the tank cannot process waste as fast.

Wipe the glass with a soft cloth or algae scraper during the water change. Algae buildup blocks light and looks bad, but it does not harm the fish. If you have a lot of algae, it usually means you are overfeeding or the tank gets too much sunlight. Reduce feeding or move the tank away from a window before you assume the glass needs constant scrubbing.

Never clean the gravel, glass, or filter all on the same day. If you do, you remove most of the bacteria at once and the tank will cycle again. Space out the tasks: clean the glass one week, rinse the filter the next, and do the water change on schedule both weeks.

Adjusting the schedule for different tank sizes and stocking levels

A heavily stocked tank — one where the fish biomass is high relative to the water volume — needs more frequent changes. A 20-gallon tank with 15 small fish produces more waste than a 20-gallon tank with 3 fish. The heavily stocked tank may need a 50 percent change every 5 to 7 days, while the lightly stocked one can go 14 days. If you are unsure whether your tank is overstocked, ask at the fish store or look up the adult size of each species you own. Fish grow, and a tank that was fine when they were juveniles may be too small once they reach full size.

Tanks under 10 gallons are harder to maintain because waste builds up faster in a smaller volume. A 5-gallon tank with two fish may need a 50 percent change every 5 days. Nano tanks (under 5 gallons) are even more demanding and are best left to experienced keepers. The smaller the tank, the less margin for error.

Planted tanks — those with live aquatic plants — can go longer between changes because plants consume some of the ammonia and nitrate. A planted 40-gallon tank might go 14 to 21 days between changes, while an unplanted one of the same size needs cleaning every 10 days. The plants do not eliminate the need to clean, but they do reduce the buildup rate.

Signs your tank needs cleaning sooner than scheduled

If the water turns cloudy or milky, a bacterial bloom is happening — usually because the tank is new or you cleaned too aggressively. Do a 25 percent water change and wait a few days. The cloudiness will clear as the bacteria settle.

If the water smells like ammonia or rotten eggs, waste is building up faster than the bacteria can handle. Do a 50 percent change when ready, then return to your normal schedule. If the smell comes back within a few days, you are overfeeding or the tank is overstocked.

If the fish are gasping at the surface or hiding constantly, ammonia or nitrate is high. Do a 50 percent change right away and test the water with a kit from the pet store. If ammonia or nitrite is above zero, do another 50 percent change the next day. Once the levels drop, return to your regular schedule but increase the frequency by one or two days.

Frequently Asked Questions

Can I use a gravel vacuum on a tank with live plants?

Yes, but carefully. Push the tube slowly into the substrate and pull it out before you suck up the roots. Vacuum around the plants rather than directly under them. If you have a lot of plants, you may not need to vacuum as often because the plants help stabilize the substrate and reduce debris buildup.

What water temperature should I use for a water change?

Match the temperature of the tank water as closely as you can — within 2 to 3 degrees. Let tap water sit in a bucket for 24 hours to reach room temperature, or use a thermometer to check it before adding it. Cold water shocks the fish and can cause illness.

Do I need to test the water to know when to clean?

No. A regular cleaning schedule based on tank size and stocking level works for most people. Water test kits are useful if the tank smells, looks cloudy, or the fish are stressed, but they are not necessary for routine maintenance. Stick to the schedule and watch the fish — they will tell you if something is wrong.

How long can I leave a tank without cleaning if I go on vacation?

A healthy, lightly stocked tank can go 2 to 3 weeks without a water change if you do not feed the fish during that time. Fish can survive weeks without food but not without water quality. Do a 50 percent change before you leave, then skip feeding entirely. Do not ask someone to feed the fish unless they know how much — overfeeding while you are gone will foul the water faster than the lack of cleaning.

Should I clean the tank before adding new fish?

Do a 25 percent water change the day before you add new fish, but do not deep-clean the tank. You want the bacteria colony intact so it can handle the extra waste from the new fish. If the tank is already clean and the water is clear, you do not need to change it at all — just add the new fish and stick to your regular schedule.