Water changes are the most important maintenance task you can do

Clean aquarium water keeps fish healthy and prevents the buildup of waste that poisons them over time. The most effective way to clean the water is a partial water change—removing 20 to 30 percent of the water and replacing it with fresh water—done once a week for most home aquariums. You do not need to empty the tank or remove the fish. The process takes 15 to 30 minutes and requires only a siphon, a bucket, and dechlorinated water.

Fish produce ammonia through their gills and waste. Beneficial bacteria in the filter convert ammonia to nitrite, then to nitrate. Nitrate builds up over time and cannot be removed by the filter—only by removing water. If you skip water changes, nitrate accumulates until it stresses or kills the fish, even though the water may look clear.

Key Takeaways

  • A partial water change of 20 to 30 percent once per week is the standard maintenance for most home aquariums and takes 15 to 30 minutes.
  • Use a siphon or aquarium vacuum to remove water from the bottom of the tank, where waste settles, rather than pouring water out from the top.
  • Always treat tap water with a dechlorinator before adding it to the tank, because chlorine in tap water kills beneficial bacteria and harms fish.
  • Match the temperature of the new water to the tank water before adding it, because a sudden temperature change stresses fish.
  • Do not clean the filter media with tap water; rinse it in old aquarium water removed during the water change to preserve the bacteria colony.

What you need to perform a water change

Gather these items before you start: a siphon (a tube that uses gravity to drain water), a clean bucket you use only for the aquarium, a dechlorinator (a liquid or tablet that removes chlorine from tap water), and a thermometer if your tank does not have one built in. A siphon is inexpensive—most aquarium stores sell them for under $15—and it is the fastest way to remove water without disturbing the gravel or the fish.

If you do not have a siphon, you can use a large cup or pitcher to scoop water out, but this takes longer and risks splashing the fish. Some aquariums come with a gravel vacuum, which is a siphon with a wide tube at the end that stirs up the gravel as it drains—this removes more waste but is not necessary for basic maintenance.

Dechlorinator comes in liquid or tablet form. Liquid dechlorinator is faster because you add it to the bucket and wait a few minutes before pouring the water in. Tablets dissolve more slowly. Both work equally well; choose whichever fits your routine. Some dechlorinators also bind heavy metals and add beneficial bacteria, but plain dechlorinator is sufficient.

How to siphon water out of the tank

Place the bucket on the floor next to the aquarium, lower than the water level of the tank. Fill the siphon tube with water by submerging it completely in the tank, then pinch one end closed with your thumb. Lift that end out of the tank and place it into the bucket while keeping your thumb over the opening. Release your thumb—water will flow down into the bucket as long as the bucket end of the tube stays lower than the tank.

Once the siphon is flowing, you can use a gravel vacuum attachment if you have one, or straightforward hold the tube near the bottom of the tank to draw out water and waste. Move the tube slowly across the gravel to stir it up and remove settled debris. Stop when you have removed about 20 to 30 percent of the water—for a 50-gallon tank, that is 10 to 15 gallons. If you are unsure how much you have removed, mark the tank glass with a piece of tape at the starting water level before you begin.

Do not drain water while the filter is running if your filter sits outside the tank (a canister or hang-on filter). Stop the filter first, then resume it after you have refilled the tank. If your filter is inside the tank (a sponge filter or internal canister), you can leave it running.

Preparing and adding fresh water

Fill the bucket with tap water and add dechlorinator according to the bottle instructions—usually a few drops per gallon. Wait the time specified on the bottle, typically 2 to 5 minutes. While you wait, check the temperature of the new water with a thermometer. It should match the tank temperature within 2 degrees Fahrenheit. If the new water is too cold, let it sit at room temperature for a few minutes or run warm (not hot) tap water into the bucket until the temperature is close.

Pour the treated water slowly into the tank. Pouring onto a plate or into your hand and letting it run off slows the flow and prevents the current from scattering gravel or stressing the fish. If you are adding more than 10 gallons, pour in stages—add half, wait a minute, then add the rest.

Turn the filter back on if you turned it off. The water level should return to where it was before the change. If it is noticeably lower, top it off with a small amount of treated water.

Cleaning the filter without killing beneficial bacteria

The filter media—the sponge, cartridge, or floss inside the filter—traps waste and houses the beneficial bacteria that convert ammonia. If you rinse the filter media in tap water, the chlorine kills the bacteria and you lose the biological filtration that keeps the tank stable. Instead, rinse the filter media in the old aquarium water you just removed.

During a water change, set aside a bucket of the water you siphoned out. After the water change is complete, swish the filter media gently in this old water to remove trapped waste. The bacteria survive because they are in aquarium water, not tap water. Squeeze the media gently—do not wring it—and return it to the filter. Discard the old water down the drain.

You do not need to clean the filter every week. Clean it only when water flow slows noticeably, usually every 2 to 4 weeks depending on how much you feed the fish and how many fish you have. A clogged filter reduces water circulation and oxygen, so do not ignore it, but over-cleaning removes bacteria and destabilizes the tank.

How often to change water based on tank size and stocking

A standard 20-gallon aquarium with a moderate number of fish benefits from a 25 percent water change once a week. A 50-gallon tank with the same fish-to-water ratio needs the same frequency. A heavily stocked tank—one where fish are crowded—may need 30 to 40 percent changes twice a week. A lightly stocked tank with few fish and a strong filter may need only a 20 percent change every 10 days.

The rule of thumb is: if the water looks cloudy, smells bad, or the fish are gasping at the surface, you are not changing water often enough. Start with once a week and adjust based on what you observe. A test kit that measures ammonia, nitrite, and nitrate can tell you exactly what is building up, but for most home aquariums, a regular weekly schedule prevents problems before they start.

Newly set up tanks (less than 4 weeks old) need more frequent changes because the beneficial bacteria colony is still establishing. Do 25 percent changes twice a week for the first month, then move to the standard once-a-week schedule.

Common mistakes that harm water quality

Using tap water without dechlorinator is the most common mistake. Chlorine kills fish directly at high concentrations and always kills the beneficial bacteria in the filter, which crashes the nitrogen cycle and causes ammonia spikes that kill fish days later. Always treat tap water before it enters the tank.

Adding water that is much warmer or colder than the tank stresses fish and can trigger disease. A 5-degree difference is noticeable to the fish; a 10-degree difference can be fatal. Match the temperature before pouring.

Cleaning the filter in tap water removes the bacteria and destabilizes the tank. Rinse filter media only in old aquarium water. Cleaning the gravel with a vacuum is good; cleaning the filter media is harmful.

Changing too much water at once—more than 50 percent—removes too many beneficial bacteria and can cause an ammonia spike. Stick to 20 to 30 percent for routine maintenance.

Frequently Asked Questions

Can I use bottled water instead of tap water?

Bottled water is safe and does not contain chlorine, so you do not need to treat it with dechlorinator. However, it is expensive for regular water changes. Tap water treated with dechlorinator is the standard and most affordable option. Use bottled water only if your tap water is heavily contaminated or if you are doing an emergency water change and do not have dechlorinator on hand.

What if I forget to add dechlorinator?

If you realize the mistake when ready after pouring, do a partial water change with treated water to dilute the chlorine. If you discover it the next day, the chlorine has usually evaporated, but the beneficial bacteria may have been damaged. Do a 25 percent water change with treated water and monitor the fish for stress. If ammonia or nitrite spikes, do another 25 percent change a few days later.

How do I know if my water change schedule is working?

Clear water and healthy fish are the main signs. Fish should be active, eat normally, and show bright colors. If the water becomes cloudy, smells fishy, or the fish gasp at the surface or hide, increase the frequency or volume of water changes. A test kit for ammonia and nitrate gives you exact numbers, but observation is usually enough for a healthy tank.

Do I need to remove the fish during a water change?

No. Removing the fish stresses them and is not necessary. The siphon removes water gently, and the fish can stay in the tank. The only exception is if you are doing a very large change (more than 50 percent) or if a fish is sick and needs isolation—in those cases, move the fish to a temporary container while you work.

Can I use a water conditioner instead of dechlorinator?

Water conditioner and dechlorinator are often the same product. Check the bottle—if it says it removes chlorine and chloramine, it is dechlorinator. Some conditioners also add beneficial bacteria or bind heavy metals, which is fine. The important thing is that it removes chlorine before the water enters the tank.