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 filters cannot catch and restores minerals your fish need. 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 run a filter — a 20-gallon tank with a filter and four fish can go longer between changes than a 10-gallon tank with eight fish and no filter.

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 using water that is too cold or too warm, which shocks the fish. The second most common mistake is removing too much water at once, which destabilizes the bacteria in your filter that keep ammonia and nitrite from poisoning the tank.

Key Takeaways

  • Remove 25 to 30 percent of your tank water using a siphon or gravel vacuum, which also cleans the substrate where waste collects.
  • Match the temperature of fresh water to your tank water within 2 degrees Fahrenheit before adding it back, using a thermometer to check both.
  • Let tap water sit for 24 hours before use if your tap water is heavily chlorinated, or use a dechlorinator product to treat it when ready.
  • Do a partial water change every one to two weeks for most tanks, but increase frequency if ammonia or nitrite readings are high.

Gather your supplies before you start

You need a clean bucket (5 to 10 gallons works for most home tanks), a siphon or gravel vacuum, a thermometer, and treated water. A siphon is a tube you fill with water and position so gravity pulls water out of the tank into the bucket. A gravel vacuum is a siphon with a wide tube at the end that stirs the substrate as it removes water, pulling out trapped waste. Both cost between $10 and $30.

If you do not have either, you can scoop water out by hand using a cup or small pitcher, but this does not clean the substrate and takes longer. Do not use a bucket that has held soap, chemicals, or anything other than water — residue will poison your fish. If you are unsure, use a new bucket from a hardware store.

Check and prepare your replacement water

Tap water contains chlorine and sometimes chloramine, which harm fish. If your tap water is lightly chlorinated, letting it sit uncovered for 24 hours allows the chlorine to evaporate. If your water utility uses chloramine (which does not evaporate), you need a dechlorinator product — brands like Seachem Prime or API Stress Coat cost $5 to $15 and treat large volumes. Follow the bottle instructions for how much to use.

The most important step is matching temperature. Fill a bucket with the amount of water you plan to add back to the tank, then measure its temperature with a thermometer. Measure your tank water temperature as well. If the difference is more than 2 degrees Fahrenheit, let the bucket sit until the temperatures match, or add ice or warm water to the bucket to adjust it. Cold water shocks fish and can trigger disease; hot water kills beneficial bacteria in the filter.

Remove the old water and clean the substrate

Place your bucket on the floor next to the tank. If you are using a siphon, fill the tube completely with water (some people use their mouth, others use a faucet), pinch both ends to keep the water in, position one end in the tank and one in the bucket, then release. Water should flow into the bucket by gravity. If it does not, the bucket is too high — lower it or place the tank on a higher surface.

If you are using a gravel vacuum, push the wide tube down into the substrate in several spots around the tank, holding it there for a few seconds each time. The vacuum will pull water and debris out together. Avoid the areas where your fish hide or where plants are rooted, since you may uproot them or stress hiding fish. Remove water until you have taken out roughly 25 to 30 percent of the tank — for a 20-gallon tank, that is about 5 to 6 gallons.

Refill the tank slowly and watch for problems

Pour the treated, temperature-matched water back into the tank slowly, over the course of a few minutes. If you pour too fast, you can uproot plants, scatter substrate, or stress fish. Some people pour the water onto a plate or into their hand to slow it down further. Watch your fish as you refill — they should behave normally. If they gasp at the surface or hide, stop and check the temperature again.

Once the tank is full, turn any filters back on if you turned them off, and leave the tank alone for a few hours. Do not feed your fish for at least an hour after a water change, since the disturbance can make them less interested in food and uneaten food will foul the water.

How often to change water based on your setup

A tank with a working filter and moderate stocking (one inch of fish per gallon of water) needs a water change every one to two weeks. A tank with no filter or heavy stocking needs a change every three to five days. A heavily planted tank with light stocking may go three weeks between changes because plants consume some of the waste.

The most reliable way to know if you are changing water often enough is to test ammonia and nitrite with a kit (brands like API Master Test Kit cost $20 to $30). If ammonia or nitrite reads above 0.25 parts per million, increase your water change frequency or reduce the number of fish. If both read 0, you can extend the time between changes slightly.

Common mistakes and how to avoid them

Using water that is too cold is the most common error — fish become lethargic and stop eating. Using water that is too hot kills the nitrifying bacteria in your filter, which can cause ammonia spikes days later. Always use a thermometer on both the tank and the replacement water.

Removing too much water at once (more than 50 percent) destabilizes the filter bacteria and can cause ammonia and nitrite to spike. Stick to 25 to 30 percent for routine changes. If you need to remove more — for example, because of a disease outbreak — do it in stages over several days, or seed the tank with filter media from an established tank to speed up bacterial recovery.

Forgetting to dechlorinate tap water will slowly poison your fish over weeks. If your water utility uses chloramine, dechlorination is not optional. If you use only chlorine, the 24-hour sit method works, but dechlorinator is faster and more reliable.

Frequently Asked Questions

Can I use bottled water instead of tap water?

Yes, but it is expensive for regular changes. Bottled water is already dechlorinated and temperature-neutral, so it works well. For a 20-gallon tank needing a 5-gallon change weekly, you would spend $5 to $10 per week on bottled water. Tap water with dechlorinator costs less than $1 per week.

What if I do not have a thermometer?

Buy one — they cost $3 to $8 and are the single most important tool for safe water changes. Without one, you risk temperature shock, which weakens fish and causes disease. A digital aquarium thermometer is more accurate than a strip thermometer.

Do I need to remove my fish before changing the water?

No. Removing fish stresses them and is unnecessary for a partial water change. Leave them in the tank and work around them. If you are doing a complete tank cleaning (which is rare), you can move them to a temporary container, but for routine 25 to 30 percent changes, they stay put.

Can I use a garden hose to refill the tank?

Only if the hose is new and has never carried pesticides or fertilizer. Most garden hoses leach chemicals into water. A bucket and pitcher are safer and slower refilling gives you time to match temperature anyway.

What should I do if my fish are gasping after a water change?

Stop feeding and check the temperature when ready — it is likely too cold or too warm. Check ammonia and nitrite if you have a test kit. If temperature is correct and ammonia is 0, do a 25 percent water change with properly treated water. If the problem persists, the issue may be something other than the water change itself.