Change 25 to 50 percent of your water every week or every two weeks

The standard schedule for most aquariums is a 25 to 50 percent water change every seven to fourteen days. This removes waste that filters cannot catch, replenishes minerals fish need, and keeps ammonia and nitrate from building up to toxic levels. The exact timing depends on how many fish you have, how much they eat, and whether your tank is planted.

A tank with heavy stocking — many fish in a small space — needs weekly changes at the 50 percent mark. A lightly stocked tank with live plants may stretch to every two weeks at 25 percent. The rule is straightforward: more fish and more waste means more frequent changes. If you skip a week and the water looks cloudy or smells, you waited too long.

Key Takeaways

  • Most aquariums need a 25 to 50 percent water change every one to two weeks, with heavily stocked tanks requiring the higher percentage and frequency.
  • Ammonia and nitrate build up between changes even in filtered tanks, and water changes are the only way to remove them completely.
  • New tanks (under four weeks old) need changes every few days because the bacterial colony that breaks down waste is not yet established.
  • Planted tanks and tanks with fewer fish can go longer between changes because plants consume nitrate and reduce the bioload.
  • Testing your water for ammonia, nitrite, and nitrate tells you whether your current schedule is working or needs adjustment.

Why filters alone are not enough

A filter removes solid waste and dead plant matter, but it does not remove dissolved chemicals. Ammonia and nitrate stay in the water even in a well-filtered tank. Fish produce ammonia through their gills and waste. Beneficial bacteria in the filter convert ammonia to nitrite, then to nitrate — but nitrate has nowhere to go except out through a water change.

Nitrate builds slowly and is less toxic than ammonia or nitrite, but it still stresses fish and promotes algae growth. After two weeks without a change, a 20-gallon tank with six goldfish will have nitrate levels that harm the fish. A water change dilutes these chemicals and gives the tank a fresh start.

New tanks need more frequent changes

A tank less than four weeks old does not yet have enough beneficial bacteria to handle fish waste safely. During this period, ammonia and nitrite spike quickly and can kill fish. Change 25 to 50 percent of the water every two to three days until the tank is established — you will see ammonia and nitrite drop to zero on a test kit, which usually takes three to six weeks.

Once ammonia and nitrite stay at zero for a week, you can move to the standard schedule. Do not skip this step. Many new aquarium owners lose fish because they assume the filter is working before the bacteria colony is large enough.

Heavily stocked tanks need weekly changes

A tank with many fish — such as a 20-gallon with ten goldfish or twenty small tropical fish — produces waste faster than the filter can process it. These tanks need a 40 to 50 percent change every seven days. If you miss a week, ammonia or nitrite may spike and stress or kill the fish.

Goldfish and plecos are heavy waste producers and need more frequent changes than tetras or guppies. If you keep large fish or many fish in a small tank, plan on weekly maintenance. Some aquarists with heavily stocked tanks do two smaller changes per week instead of one large one, which keeps the water more stable.

Planted tanks and light stocking allow longer intervals

Live plants consume nitrate as fertilizer, so a planted tank with moderate lighting and regular fertilizer dosing can go two weeks between 25 percent changes. The plants also produce oxygen and reduce the bioload by consuming some of the ammonia fish produce. A lightly stocked planted tank — four or five small fish in a 30-gallon — may need changes only every two weeks.

Even in a planted tank, do not skip changes entirely. Plants slow the buildup of nitrate but do not stop it, and they do not remove ammonia or nitrite. Test the water every two weeks to confirm that nitrate is staying below 40 parts per million. If it climbs higher, increase the change frequency or reduce the number of fish.

How to test whether your schedule is working

A water test kit tells you whether your current change schedule is adequate. Test for ammonia, nitrite, and nitrate. Ammonia and nitrite should always be zero in an established tank. Nitrate should stay below 40 parts per million, though some fish tolerate higher levels.

If ammonia or nitrite is above zero, increase your change frequency or reduce the number of fish. If nitrate is climbing above 40 between changes, do larger changes or change more often. A test kit costs eight to fifteen dollars and lasts for fifty to one hundred tests. It is the only way to know whether you are changing water too often or not often enough.

Water change steps that matter

Use a siphon or gravel vacuum to remove water from the bottom of the tank, where waste settles. This removes more waste than straightforward scooping water from the top. Vacuum the gravel gently to avoid uprooting plants or disturbing fish.

Replace the water with dechlorinated water at the same temperature as the tank. Tap water contains chlorine, which kills beneficial bacteria and harms fish. Let tap water sit for 24 hours before adding it, or use a dechlorinator product that works when ready. Cold water shocks fish and can trigger disease, so match the temperature as closely as you can — within two or three degrees.

Frequently Asked Questions

Can I do one large water change instead of two small ones?

Yes, but two smaller changes are gentler on fish. A single 50 percent change removes half the dissolved minerals and pH-buffering capacity at once, which can stress fish. Two 25 percent changes over the week keep the water more stable. For heavily stocked tanks, split the change if you can.

What if I miss a week?

Do the change as soon as you remember. Do not skip it or double up the next week. If the water looks cloudy or smells, do a 50 percent change when ready and test for ammonia and nitrite. Missing one week rarely kills fish in an established tank, but it stresses them and can trigger disease.

Do I need to change water if I have a powerful filter?

Yes. Even the best filter cannot remove dissolved nitrate. Filters remove solid waste and convert ammonia to nitrate, but only a water change removes nitrate itself. A powerful filter lets you go slightly longer between changes, but it does not eliminate the need for them.

Should I change water more often in summer?

Yes, if the water temperature rises above 78 degrees. Warm water holds less oxygen, and fish produce more waste in warm conditions. Increase your change frequency by 25 to 50 percent during hot months, or use a fan or chiller to keep the temperature stable.

Can tap water chemicals hurt my fish?

Chlorine in tap water kills beneficial bacteria and irritates fish gills. Chloramine, used in some cities, is harder to remove and requires a dechlorinator product. Let tap water sit for 24 hours to remove chlorine, or add dechlorinator before pouring it in. Check your local water report to see whether your tap water contains chloramine.