Do a water change by removing 25 to 30 percent of the tank water, then refilling with fresh water at the same temperature

A water change removes waste that the filter cannot catch and restores minerals that fish consume. Most tanks need a partial water change every one to two weeks. The exact timing depends on tank size, how many fish you have, and whether you have live plants — a planted tank can go longer between changes because plants consume some of the waste.

The process takes 15 to 30 minutes and requires only a bucket, a siphon or gravel vacuum, and a thermometer. The most common mistake is changing too much water at once or using water that is a different temperature, both of which shock the fish and can kill them.

Key Takeaways

  • Remove 25 to 30 percent of the tank water every one to two weeks using a siphon or gravel vacuum.
  • Match the temperature of the new water to the tank water within one or two degrees before adding it.
  • Let tap water sit for 24 hours or use a dechlorinator product if your tap water is chlorinated, because chlorine kills the bacteria that keep the tank stable.
  • Turn off the heater and filter before you start, then turn them back on after the tank refills.
  • Vacuum the gravel during the water change to remove fish waste that settles on the bottom.

Gather your supplies before you start

You need a clean bucket that holds at least 5 gallons, a siphon or gravel vacuum, a thermometer, and treated water. Do not use a bucket that has held soap, chemicals, or anything other than water — residue will poison the tank.

A siphon is a tube that uses gravity to pull water out of the tank into the bucket. A gravel vacuum is a siphon with a wide tube at the end that stirs up the gravel as it removes water, pulling out waste that settles on the bottom. A gravel vacuum is the better choice because it cleans the substrate at the same time. Both cost between $15 and $40.

If your tap water is chlorinated, you have two options: let the water sit in an open bucket for 24 hours so the chlorine evaporates, or add a dechlorinator product like Seachem Prime or API Stress Coat. Dechlorinator works when ready and costs $8 to $15 per bottle, which treats hundreds of gallons. If you are unsure whether your tap water is chlorinated, call your local water utility — they can tell you.

Remove the old water and clean the gravel

Turn off the heater and filter. If you leave them running while the water level drops, the heater can overheat and the filter intake can suck air, which damages the motor.

Place the bucket on the floor next to the tank. If you are using a siphon, fill the tube with water by submerging it in the tank, then place one end in the bucket — gravity will pull the water out. If you are using a gravel vacuum, place the wide end on the gravel and push down gently; the suction will start the flow.

Remove water until the tank is 25 to 30 percent empty. For a 20-gallon tank, that is 5 to 6 gallons. For a 55-gallon tank, that is 14 to 16 gallons. If you have a gravel vacuum, move it slowly across the bottom of the tank as you remove water — this pulls out the dark waste that accumulates under the gravel.

Refill with treated water at the correct temperature

Check the temperature of the water in the tank with a thermometer. Most tropical fish tanks run between 76 and 80 degrees Fahrenheit. Measure the temperature of your replacement water and adjust it until it matches the tank within one or two degrees.

If your replacement water is too cold, run it under warm tap water or let it sit in a warm room for a few hours. If it is too hot, add ice cubes or let it cool. Do not pour hot water directly into the tank — the sudden temperature change will shock the fish.

Pour the water slowly into the tank. If you pour too fast, you can uproot plants, scatter the gravel, and stress the fish. Pouring into a cup or bowl placed on the gravel slows the flow and spreads the water evenly.

Turn equipment back on and monitor the tank

Once the tank is full, turn the heater and filter back on. The heater will take 30 minutes to an hour to bring the water back to the correct temperature if it cooled during the change.

Watch the fish for the next hour. They may hide or move slowly for a few minutes after the change — this is normal. If they gasp at the surface, dart erratically, or show signs of stress after 10 minutes, the water temperature or chemistry is wrong. Turn off the heater and filter again and do a second partial change with water that matches the tank more closely.

Adjust your schedule based on tank conditions

A tank with heavy fish load, no plants, or a small filter may need a water change every week. A planted tank with few fish and a strong filter may only need a change every three weeks. Watch the water clarity and fish behavior — if the water looks cloudy or the fish seem lethargic, do a change sooner.

Keep a log of when you do changes and what the tank looked like that day. Over time you will see patterns and can predict the best schedule for your setup. Most aquarists find that a consistent schedule is easier to remember than waiting for visible signs of trouble.

Frequently Asked Questions

Can I use bottled water instead of tap water?

Yes, but it is expensive and unnecessary for most tanks. Bottled water lacks minerals that fish need, so you would have to add them back. Tap water with dechlorinator is cheaper and works better. Bottled water is useful only if your tap water is very hard or has high nitrates that your filter cannot remove.

What if I change the water and the fish die?

The most common cause is temperature shock — water that is too cold or too hot. The second most common cause is chlorine in untreated tap water. Check the temperature of the water you added and whether you used dechlorinator. If you did both correctly, the problem may be something else in the tank, not the water change itself.

Do I need to remove the fish before I change the water?

No. Removing the fish stresses them more than a slow, careful water change does. Leave them in the tank and work slowly. The only exception is if you are cleaning the tank completely, which you should do only once every few months if at all.

How often should I change the water in a new tank?

A new tank with fish in it should have a 25 percent change every three to four days for the first month, because the filter bacteria are still building up and cannot handle the waste load. After the tank is established, move to your normal schedule of every one to two weeks.