Change 25 to 50 percent of your aquarium water every one to two weeks

The standard schedule for most aquariums is a partial water change of 25 to 50 percent every 7 to 14 days. This removes waste products that accumulate between changes while keeping the tank stable enough that fish do not experience shock from sudden chemistry shifts. The exact timing depends on tank size, how many fish you have, what you feed them, and whether your filter is working well.

Smaller tanks need more frequent changes because waste builds up faster in less water. A 10-gallon tank with five fish may need a 50 percent change every week, while a 55-gallon tank with the same fish might go two weeks between 25 percent changes. Overstocked tanks — tanks with more fish than the filter can handle — need changes every 3 to 5 days to keep ammonia and nitrite from spiking.

Never do a 100 percent water change unless the tank has crashed (ammonia or nitrite has become toxic and fish are dying). A complete drain removes beneficial bacteria that live in the gravel and filter, and it shocks the fish with a sudden chemistry change. Even when cleaning the filter or gravel, leave some of the old water in the tank.

Key Takeaways

  • Most tanks need a 25 to 50 percent water change every 7 to 14 days, with smaller tanks and overstocked tanks needing changes more often.
  • Partial changes remove waste while keeping the tank chemistry stable; complete drains kill beneficial bacteria and stress fish.
  • Test ammonia and nitrite levels if you are unsure whether your schedule is working — levels above 0.5 ppm mean you need to change water more often.
  • Tap water needs to sit for 24 hours or be treated with dechlorinator before adding it to the tank, because chlorine kills beneficial bacteria.
  • Gravel vacuuming during water changes removes settled waste and reduces the buildup that makes tanks foul between changes.

Why water changes matter more than filter strength alone

A filter removes solid waste and some chemicals, but it cannot remove all the nitrogen compounds fish produce. Fish excrete ammonia through their gills and in their waste. Bacteria in the filter convert ammonia to nitrite, then to nitrate. Nitrate does not harm fish at low levels, but it accumulates over time and makes the water acidic and cloudy. Water changes are the only way to remove nitrate.

Without regular changes, nitrate builds to 40, 60, or 80 ppm — levels that stress fish, slow their growth, and make them vulnerable to disease. Plants in the tank will use some nitrate, but not enough to replace water changes. Even a powerful filter cannot prevent this buildup because the filter is not removing nitrate; it is only converting ammonia into it.

How tank size and stocking level change your schedule

A heavily stocked tank — one where fish occupy most of the visible space — produces waste faster than the filter can process it. In these tanks, ammonia and nitrite can spike between changes, poisoning the fish. If your tank looks crowded, test the water twice a week. If ammonia or nitrite reads above 0.5 ppm, change water every 3 to 5 days instead of weekly.

A lightly stocked tank with a strong filter may go two weeks between changes without ammonia or nitrite rising. The rule of thumb is one inch of fish per gallon of water, though this varies by species — goldfish need more space than tetras. If you are unsure whether your tank is overstocked, count the inches of fish and compare to your tank volume.

Tanks with live plants can sometimes go longer between changes because plants consume nitrate and ammonia. A heavily planted 20-gallon tank might need changes only every two to three weeks. A bare 20-gallon tank with the same fish would need changes every week.

Steps for a safe partial water change

Turn off the heater and filter before you start. Use a gravel vacuum — a tube with a bucket attachment — to siphon water from the bottom of the tank while stirring the gravel. This removes settled waste and dead plant matter. Vacuum about 25 to 50 percent of the tank, depending on your schedule.

While the tank is draining, prepare the new water. If you are using tap water, let it sit in a bucket for 24 hours so chlorine evaporates, or add a dechlorinator product and wait 15 minutes. The new water should be the same temperature as the tank water — use a thermometer to check. Cold water shocks fish and can trigger disease.

Pour the new water slowly into the tank, aiming it at a plate or into your hand so it does not blast the gravel and disturb the fish. Turn the filter and heater back on once the tank is full. The whole process takes 20 to 30 minutes for a standard tank.

Signs your water change schedule is not working

If the water turns cloudy or smells like ammonia within a few days of a change, you are not changing often enough. Cloudy water usually means a bacterial bloom — bacteria multiplying because there is too much ammonia for the filter to handle. Ammonia smell means ammonia is building up faster than bacteria can convert it.

Fish gasping at the surface, hiding constantly, or refusing food are signs of poor water quality. Test the water with a kit that measures ammonia, nitrite, and nitrate. Ammonia and nitrite should both be 0 ppm in an established tank. Nitrate should stay below 40 ppm. If either ammonia or nitrite is above 0.5 ppm, increase the frequency of your changes when ready.

Algae blooms — green water or thick algae on the glass — usually mean nitrate is high and light is strong. This is not an emergency, but it signals that water changes are not keeping up with waste production. Increase changes to weekly if you are doing them every two weeks, or add more plants to consume the excess nitrate.

Dechlorinator and water conditioner: what they do

Tap water contains chlorine to kill bacteria for human use, but chlorine also kills the beneficial bacteria in your filter. If you add untreated tap water to your tank, you will set back the nitrogen cycle and ammonia will spike. A dechlorinator is a chemical that neutralizes chlorine in seconds. Brands like Seachem Prime and API Stress Coat are common and inexpensive.

Some dechlorinators also bind heavy metals and ammonia, which is useful if your tap water is hard or has high ammonia. Read the label to see what the product does. If you let tap water sit for 24 hours in an open bucket, chlorine will evaporate on its own, so dechlorinator is optional but faster and more reliable.

Established tanks versus new tanks

A new tank that has been running for less than four weeks does not yet have enough beneficial bacteria to handle fish waste. During the first month, do a 25 percent change every 3 to 5 days, and test ammonia and nitrite daily. Once ammonia and nitrite both stay at 0 ppm for a week, the tank is cycled and you can move to a normal schedule.

An established tank with a mature filter can sometimes go longer between changes if it is lightly stocked. But even established tanks need changes at least every two weeks. Nitrate still accumulates, and no filter removes it completely. Skipping changes for a month will cause nitrate to climb and water quality to decline.

Frequently Asked Questions

Can I use bottled water instead of tap water?

Bottled water works but is expensive for regular changes. Distilled water lacks minerals fish need and can make the water too soft. If your tap water is very hard or has high chlorine, bottled water is an option for small tanks, but for most people tap water with dechlorinator is the practical choice.

What if I miss a water change?

One missed change usually does not harm the tank, but ammonia and nitrite may start to rise. Do the change as soon as you remember. If you regularly forget, set a phone reminder or move your change day to a time that fits your routine better.

Do I need to test the water before every change?

No. Test once a month in an established tank to make sure ammonia and nitrite are at 0 ppm and nitrate is below 40 ppm. If levels are normal, your schedule is working. Test more often if you see signs of poor water quality or if the tank is new.

Is it okay to change water during feeding time?

Yes, but do the change before you feed or several hours after. Uneaten food decays and makes water quality worse, so removing it during a water change helps. Feeding right before a change means the food sits in the tank while you work, which defeats the purpose.

How do I know if my filter is strong enough?

A filter should turn over the entire tank volume at least three times per hour. A 20-gallon tank needs a filter rated for 60 gallons per hour or higher. If ammonia or nitrite stays above 0 ppm even with weekly changes, your filter is too weak for the number of fish you have.