Water changes keep your tank clean and your fish healthy

A water change means removing some of the old water from your tank and replacing it with fresh water. You do this regularly—usually once a week for most tanks—because fish waste and uneaten food break down into ammonia and nitrate, which build up over time even with a filter running. Changing 25 to 50 percent of the water dilutes these waste products and keeps the tank environment stable.

The process itself is straightforward: drain some water out, add fresh water back in at the same temperature, and you're done. The tricky part is doing it without shocking your fish or disrupting the beneficial bacteria living in your filter and substrate. Temperature swings and sudden water chemistry changes stress fish and can kill them, so the goal is to make the transition gradual and gentle.

Key Takeaways

  • Remove 25 to 50 percent of your tank water once a week, depending on tank size and how many fish you have.
  • Match the temperature of new water to the old water before adding it to the tank, using a thermometer to check both.
  • Let tap water sit for 24 hours before use, or treat it with a dechlorinator product to remove chlorine that can harm fish.
  • Use a siphon or aquarium vacuum to remove water from the bottom where waste settles, rather than pouring water out from the top.
  • Add new water slowly—pour it gently or use an airline tube to trickle it in—so fish don't get knocked around by the current.

Gather your supplies before you start

You need a bucket (dedicated to the aquarium, never used for cleaning chemicals), a thermometer, a siphon or aquarium vacuum, and treated water. A siphon is a tube that uses gravity to drain water; an aquarium vacuum is a larger tube with a bulb that sucks water and debris from the substrate at the same time. Both work well—the vacuum is faster and cleans the gravel, but the siphon is cheaper and simpler.

For the water itself, tap water contains chlorine and sometimes chloramine, which damage fish gills. You have two options: let tap water sit in an open bucket for 24 hours so chlorine evaporates naturally, or add a dechlorinator product (brands like Seachem Prime or API Stress Coat are common) according to the bottle's instructions. Dechlorinator works when ready, so it's faster if you're doing a water change on short notice.

If your tap water is very hard or very soft, or if you have sensitive fish like discus or cardinal tetras, you may need to adjust the water chemistry. Most community fish tolerate tap water fine. Check your fish species' needs if you're unsure.

Match the temperature before adding new water

Temperature shock is one of the biggest stressors in a water change. Fish are cold-blooded, so a sudden drop or rise in water temperature can slow their metabolism, weaken their immune system, and sometimes kill them outright. Use a thermometer to check the temperature of the water already in your tank, then adjust your new water to match it as closely as you can.

If your new water is too cold, run it under warm tap water or let it sit in a warm room for a while. If it's too hot, add cold water or let it cool. Aim for within 1 to 2 degrees Fahrenheit of the tank temperature. This takes a few minutes but prevents a lot of stress.

Drain the old water using a siphon or vacuum

Place your bucket on the floor next to the tank—it needs to be lower than the tank for gravity to work. Insert the siphon tube into the tank and fill it with water by sucking on the end (or using the priming bulb if your siphon has one). Once water flows, gravity pulls it down into the bucket. Position the tube end in the bucket and let it drain until you've removed the amount you want—usually 25 to 50 percent of the tank volume.

If you're using an aquarium vacuum instead, push the wide tube down into the gravel, squeeze the bulb to start the siphon, and move the tube slowly across the bottom of the tank. The vacuum sucks up debris and water at the same time, which cleans the substrate as you drain. This is especially useful if you see brown or black spots in the gravel, which are pockets of decaying waste.

Don't drain more than 50 percent unless you're treating a specific problem like an algae bloom or a disease outbreak. Draining too much removes the beneficial bacteria in the water column and can crash your nitrogen cycle.

Add the new water slowly and gently

Pour the new water into the tank slowly, or better yet, trickle it in using an airline tube (a thin plastic tube connected to an air pump). Pouring fast creates a current that can stress fish and stir up debris you just cleaned. Trickling takes longer but is gentler. If you don't have an airline tube, pour into a small cup or bowl placed in the tank so the water spreads out rather than hitting one spot.

Watch your fish as you add water. They should stay calm and continue their normal behavior. If they dart around or hide, you're adding water too fast or the temperature is off—stop and wait a few minutes before continuing.

Check your filter and heater after the change

Once the water level is back to normal, make sure your filter intake and heater are still submerged and working. If your filter intake is above the water line, it will suck air and stop working. If your heater is exposed, it can overheat and crack. Both should be fully covered by water.

Turn the filter back on if you turned it off during the change (some people do this to avoid stirring up debris, though it's not necessary). Check that water is flowing out of the filter outlet and that the heater is warming the water back to the right temperature. This usually takes 30 minutes to an hour.

How often to change water depends on your tank

A lightly stocked tank with good filtration may only need a 25 percent change once a week. A heavily stocked tank or one with large, messy fish like goldfish or plecos may need 50 percent changes twice a week. Tanks with no filter (which is not recommended) need 50 percent changes every few days.

Watch your water clarity and smell. If the water looks cloudy or smells like ammonia, you're not changing water often enough. If it stays clear and smells fresh, your schedule is working. You can also test the water with an aquarium test kit to measure ammonia, nitrite, and nitrate levels—if any of these are high, increase the frequency or volume of your changes.

Frequently Asked Questions

Can I use bottled water or distilled water instead of tap water?

Bottled water is fine if it's mineral water or spring water, but distilled water lacks minerals your fish need and can cause health problems over time. Tap water treated with dechlorinator is usually the best choice. If your tap water is very poor quality, ask at an aquarium store about water conditioners or reverse osmosis systems.

What if I forget to dechlorinate the water?

Chlorine at tap levels won't kill fish when ready, but it irritates their gills and stresses them. If you realize you forgot, do a partial water change with dechlorinated water the next day to dilute the chlorine. Going forward, treat the water before adding it. It takes 30 seconds and prevents problems.

Should I turn off the filter during a water change?

You don't have to, but many people do to avoid stirring up settled debris. If you turn it off, leave it off for only 10 to 15 minutes—longer than that and the beneficial bacteria in the filter media can start to die. Turn it back on once you've finished adding new water.

How do I know if the water temperature is right?

Use a thermometer to measure both the tank water and the new water before mixing them. Aim for the new water to be within 1 to 2 degrees of the tank temperature. If you don't have a thermometer, let new water sit in a room at the same temperature as your tank for a few hours before use.

Can I change all the water at once?

No. Changing 100 percent of the water removes all the beneficial bacteria that live in the tank and filter, which crashes the nitrogen cycle and can kill your fish. Only do a full water change if you're treating a serious disease or algae outbreak, and even then, save some of the old filter media to restart the cycle. For routine maintenance, stick to 25 to 50 percent changes.