Do a partial water change every one to two weeks

A partial water change means removing some of the old water and replacing it with fresh water. This is the most common maintenance task and the one that keeps your tank stable. Most aquariums need 25 to 30 percent of the water replaced each week, though tanks with heavy fish loads or no plants may need 40 to 50 percent.

The goal is to remove waste that filters cannot catch and replenish minerals that fish consume. Skipping water changes lets ammonia and nitrate build up, which stresses fish and causes disease. A consistent schedule — say, every Sunday morning — is easier to remember than waiting until the water looks dirty.

Key Takeaways

  • Remove 25 to 30 percent of the water weekly using a siphon or gravel vacuum, which also pulls debris from the bottom.
  • Let tap water sit for 24 hours or treat it with 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 fish.
  • Do not remove the filter media or substrate during a water change, as this kills 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 that uses gravity to drain water into a bucket. A gravel vacuum is a siphon with a wide tube at the end that sucks debris from the substrate as it drains — this is the better choice because it cleans the bottom at the same time.

You also need a bucket (dedicated to the aquarium, not used for cleaning chemicals), a thermometer to check water temperature, and dechlorinator if your tap water is chlorinated. Many municipal water systems add chlorine to kill bacteria for human use, but it kills the beneficial bacteria in your tank. Dechlorinator comes as a liquid or powder and costs a few dollars per bottle.

If you do not have dechlorinator, you can let tap water sit in an open bucket for 24 hours — chlorine evaporates on its own. This is slower but free.

Remove the old water using a siphon or gravel vacuum

Place your bucket on the floor below the tank level. If you are using a siphon, submerge the tube in the tank, pinch one end, and lower the other end into the bucket — water will flow downward by gravity. If you are using a gravel vacuum, push the wide tube into the substrate and let the suction pull water and debris up through the tube.

Remove only the amount you planned — if you decided on 25 percent, drain until the water level drops by one quarter. Do not drain below the bottom of any decorations or plants, as this can expose beneficial bacteria to air. Work slowly so you do not disturb fish or uproot plants.

While the water drains, this is a good time to wipe algae off the glass with a soft cloth or algae scraper. Do not use soap or chemicals — just water and a non-abrasive pad.

Prepare the replacement water before adding it back

If you have dechlorinator, fill your bucket with tap water and add the dose listed on the bottle — usually a few drops per gallon. If you are using the 24-hour method, use water you prepared the day before. Either way, check the temperature with a thermometer.

The new water should be within 2 to 3 degrees of the tank temperature. If it is much colder, let it sit longer or add warm water from the tap (test it first). If it is much warmer, add cold water or let it cool. Fish are sensitive to sudden temperature swings, and a large change can trigger stress or disease.

Once the temperature matches and the dechlorinator has been added, the water is ready to go back in.

Pour the new water back into the tank slowly

Pour the prepared water into the tank gradually, not all at once. If you pour too fast, you can disturb the substrate, uproot plants, or create a current that stresses fish. A slow pour also gives the water time to mix with the existing water and reach the same temperature throughout the tank.

Some people place a plate or bowl on the substrate and pour onto that to break the force of the water. This is helpful if you have delicate plants or a sandy bottom. Once the bucket is empty, the water change is complete.

Do not clean the filter or substrate during a water change

The filter media and substrate are home to beneficial bacteria that convert fish waste into less toxic compounds. If you rinse the filter or replace the substrate during a water change, you kill most of these bacteria, and the tank can become toxic while the bacteria population rebuilds.

Clean the filter only when water flow slows noticeably — usually every two to four weeks. When you do, rinse the filter media in old tank water (not tap water) to preserve the bacteria. Replace substrate only when it becomes compacted or foul-smelling, which can take months or years depending on the tank size and fish load.

Adjust your schedule based on tank conditions

A weekly 25 to 30 percent change works for most tanks, but some situations call for more frequent or larger changes. Tanks with many fish, no plants, or no filter need 40 to 50 percent changes weekly or even twice weekly. Heavily planted tanks with few fish may need only 20 percent every two weeks.

Watch your fish behavior and water clarity. If fish gasp at the surface, hide, or show torn fins, the water quality is poor and you need larger or more frequent changes. If the water stays clear and fish are active, your current schedule is working. You can also test ammonia and nitrate with an inexpensive kit if you want to measure water quality directly.

Frequently Asked Questions

Can I use bottled water instead of tap water?

Bottled water works but is expensive for regular water changes. If your tap water is very hard or heavily chlorinated, bottled water is an option for small tanks. For larger tanks, dechlorinator is cheaper. Some people use a mix — bottled water for the first change and tap water with dechlorinator after that.

What if I forget a water change for a week or two?

One missed change usually does not harm fish, but two or more in a row can let ammonia and nitrate build up. When you do the next change, do 40 to 50 percent instead of 25 to 30 percent to remove the extra waste. Then return to your normal schedule. If fish show signs of stress — gasping, hiding, or clamped fins — do a large change right away.

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

No. Leaving the filter on is fine and actually helpful because it keeps water moving and prevents dead spots. If the water level drops below the filter intake, turn it off temporarily so it does not run dry, then turn it back on once the new water is added.

Can I use a garden hose to fill the tank?

Only if the hose is new and has never carried pesticides or fertilizer. Even then, rinse it thoroughly first. Most garden hoses leach chemicals into water, so a clean bucket is safer. If you use a hose, treat the water with dechlorinator afterward.

How do I know if my tap water needs dechlorinator?

If your tap water smells like a swimming pool, it is chlorinated. Call your local water utility and ask — they can tell you whether chlorine is added. If you are unsure, use dechlorinator anyway. It is inexpensive and does no harm if chlorine is not present.