Why and how often to change your aquarium water

Water changes remove waste that builds up even in a filtered tank—fish produce ammonia, uneaten food decays, and plant matter breaks down. These create conditions that stress fish and damage their gills. A partial water change (removing and replacing 25 to 50 percent of the water) dilutes these waste products and restores the chemical balance your fish need.

How often you change water depends on your tank size, how many fish you have, and whether you have live plants. A small tank with many fish needs changes every week or two. A larger, planted tank with fewer fish may need changes every two to four weeks. If your tank is heavily stocked or has no plants, weekly changes are safer. Watch your water clarity and fish behavior—if the water looks cloudy or fish gasp at the surface, change water sooner.

Key Takeaways

  • Remove 25 to 50 percent of the water using a siphon or gravel vacuum, which also cleans debris from the bottom.
  • Treat tap water with a dechlorinator before adding it back, because chlorine harms fish and beneficial bacteria.
  • Match the temperature of new water to the tank water within a few degrees to avoid shocking the fish.
  • Small tanks need water changes every one to two weeks; larger tanks may go two to four weeks between changes.
  • Never remove all the water at once unless you are cleaning the tank completely, because you will lose the bacteria that break down waste.

Gather your supplies before you start

You will need a siphon or gravel vacuum to remove water. A siphon is a straightforward tube you fill by hand and place so water flows downward into a bucket. A gravel vacuum is a wider tube with a bulb or electric pump that sucks water while also pulling debris from the gravel—this is easier and cleans the substrate at the same time. Both are inexpensive and available at any aquarium store.

You also need a bucket (dedicated to the aquarium, never used for chemicals or cleaning products), a water dechlorinator (liquid or powder that removes chlorine and chloramine from tap water), and a thermometer if your tank does not have one built in. Some people use a water conditioner that does more than remove chlorine—it can bind heavy metals and reduce stress—but dechlorinator alone is sufficient. Keep these supplies near your tank so you do not forget them mid-change.

Remove the old water safely

Unplug any heaters or air pumps before you start, so you do not damage them if the water level drops suddenly. If you are using a siphon, fill the tube with water from the tank, pinch both ends to keep the water inside, place one end in the tank and the other in your bucket below tank level, then release. Water will flow downward into the bucket by gravity.

If you are using a gravel vacuum, place the wide end on the gravel at the bottom of the tank and squeeze the bulb or turn on the pump. The vacuum will pull water and debris upward through the tube into the bucket. Move slowly across the gravel, especially in corners and under decorations where waste collects. Remove 25 to 50 percent of the water—for a 20-gallon tank, that is 5 to 10 gallons. Do not empty the tank completely unless you are doing a full cleaning, because the gravel and filter contain beneficial bacteria that keep the water safe for fish.

Prepare and add the new water

Fill your bucket with tap water and add dechlorinator according to the bottle's instructions—usually a few drops per gallon. Let the water sit for a few minutes so the dechlorinator can work. Check the temperature with a thermometer; it should be within 2 to 3 degrees of your tank water. If the new water is much colder, let it warm to room temperature or run it under warm (not hot) tap water while it is still in the bucket.

Pour the treated water slowly into the tank. If you pour too fast, you will stir up the gravel and cloud the water, and you may displace decorations or plants. Pouring onto a plate or into your hand as the water enters the tank slows it down. Once you have added the water, plug your heater and air pump back in and check that your filter is running.

Clean the filter during water changes

Your filter traps waste and houses the bacteria that break down ammonia. Rinse the filter media (the sponge, cartridge, or floss inside) in old aquarium water that you removed—never use tap water, because chlorine kills the bacteria. Squeeze or swish the media gently in the bucket of old water until it looks cleaner, then return it to the filter.

If your filter has a mechanical stage (a sponge or floss that catches particles) and a biological stage (media that holds bacteria), clean only the mechanical stage during routine changes. Leave the biological media alone unless it is clogged. Some filters have a cartridge that combines both; in that case, rinse it lightly and replace it only when water flow slows noticeably, usually every four to six weeks. Replacing the filter too often removes the bacteria and forces the tank to cycle again.

What to do if your tap water is hard or has high chloramine

Some municipal water systems use chloramine instead of chlorine, and some tap water is naturally hard (high in minerals). Standard dechlorinator removes chlorine but not always chloramine; check your dechlorinator label to confirm it handles both. If your water is very hard and your fish prefer soft water (like tetras or discus), you may need to use distilled water mixed with tap water, or install a reverse-osmosis filter on your tap. This is not urgent for most community fish, but it matters if you keep sensitive species.

If you are unsure what is in your tap water, contact your local water utility—they publish a water quality report annually and can tell you the chlorine type, hardness, and pH. This information helps you choose the right dechlorinator and decide whether you need to adjust your water before adding it to the tank.

Signs you are changing water too often or not often enough

If you change water more than once a week in a mature tank, you may be removing too much beneficial bacteria and destabilizing the tank. Fish in an overly clean tank sometimes show stress because the water chemistry swings too much between changes. If you change water less than once a month in a small tank, ammonia and nitrate build up and the water becomes cloudy or smells stale.

Watch your fish for signs of stress: gasping at the surface, clamped fins, or hiding constantly. Watch the water itself: if it stays clear and has no smell, your change schedule is working. If it clouds within days or smells like rotten eggs (a sign of anaerobic bacteria in the gravel), increase the frequency or the volume of water you remove. Keep a straightforward log—write the date and how much water you changed—so you can spot patterns over a few weeks.

Frequently Asked Questions

Can I use bottled water instead of tap water?

Bottled water works but is expensive for regular changes. Distilled water is safe but lacks minerals fish need. If your tap water is very poor quality, mix bottled or distilled water with tap water rather than using it alone. For most tanks, treated tap water is the standard choice.

What if I forget to add dechlorinator?

Chlorine harms fish gills and kills beneficial bacteria. If you forget, do a small water change (10 to 15 percent) with treated water the next day to dilute the chlorine. In the future, treat your water before adding it. Some people keep a pre-filled bucket of treated water sitting overnight so it is ready when they need it.

Do I need to turn off the filter during a water change?

No. Leaving the filter running is fine and actually helps mix the new water. If your filter intake is very close to the gravel and you are using a gravel vacuum, you may briefly stir up debris that clogs the intake, but this is rare. Turn the filter off only if you are doing a full tank cleaning.

How do I know if my water is the right temperature?

Use a thermometer to check both the tank and the new water. Aim for the new water to be within 2 to 3 degrees of the tank. If it is colder, let it warm to room temperature before adding it. If it is warmer, let it cool or add a small amount of cool water to the bucket. Temperature swings stress fish and can trigger disease.

Can I do a 100 percent water change to clean the tank faster?

A 100 percent change removes all the beneficial bacteria and forces the tank to cycle again, which takes weeks. Fish will experience ammonia spikes and may die. Do a full change only if the tank is truly filthy or if you are treating a serious disease. For routine cleaning, stick to 25 to 50 percent changes weekly or every two weeks.