Water changes are the most important maintenance task you can do

The fastest way to keep your aquarium healthy is to remove some of the old water and replace it with fresh water on a regular schedule. Fish produce waste that breaks down into ammonia and nitrate—chemicals that build up over time and stress or kill your fish, even if the water looks clear. A partial water change removes these invisible toxins before they accumulate to dangerous levels.

Most aquariums need a 25 to 30 percent water change once a week. Tanks with more fish, less filtration, or live plants may need changes twice a week. Tanks with heavy filtration and few fish might go two weeks between changes. The goal is to keep ammonia and nitrate low enough that your fish stay active and eat normally.

Key Takeaways

  • Change 25 to 30 percent of your tank water once a week by siphoning out old water and adding fresh water of the same temperature.
  • Treat tap water with a dechlorinator before it enters the tank, because chlorine and chloramine in tap water kill beneficial bacteria and harm fish gills.
  • Let new water sit in a bucket for 15 to 30 minutes so it reaches room temperature and matches the tank temperature when you pour it in.
  • Never remove the filter media or scrub the gravel aggressively during a water change, because you will destroy the bacteria colony that breaks down fish waste.
  • Test your water with a kit once a week to track ammonia and nitrate levels, so you know if you need to change water more or less often.

Gather your equipment before you start

You need a siphon (a tube that uses gravity to pull water out), a clean bucket, a dechlorinator (a liquid or powder that removes chlorine from tap water), and a thermometer. A gravel vacuum—a siphon with a wide tube at the end—lets you remove water and stir the gravel at the same time, which pulls out trapped waste. A basic gravel vacuum costs $15 to $40 and will last for years.

Keep a bucket used only for the aquarium; do not use one that held cleaning chemicals or food. A 5-gallon bucket is standard for most home tanks. Buy dechlorinator at any aquarium store or online—brands like Seachem Prime and API Stress Coat are common and cost $8 to $15 per bottle. One bottle lasts months because you use only a few drops per gallon.

Remove the old water safely

Start the siphon by submerging the tube completely in the tank, then place your thumb over one end and lower that end into the bucket. Release your thumb and water will flow out. If you are using a gravel vacuum, push the wide tube down into the gravel as you siphon—this stirs up waste and pulls it out along with the water. Work slowly so you do not stress the fish.

Remove water until the bucket is full or you have taken out about 25 to 30 percent of the tank. If your tank holds 20 gallons, remove 5 to 6 gallons. If it holds 40 gallons, remove 10 to 12 gallons. Do not drain more than one-third of the tank at once, because you will remove too many beneficial bacteria and cause ammonia spikes that harm fish.

While the water drains, look at what comes out. If you see a lot of brown or black sludge in the bucket, your gravel needs more frequent stirring. If the water is very cloudy, your filter may be clogged and needs cleaning.

Prepare fresh water and add dechlorinator

Fill your bucket with tap water at room temperature. Add dechlorinator according to the bottle instructions—usually 2 to 3 drops per gallon. Stir the bucket for 10 seconds to mix it in. Let the water sit for 15 to 30 minutes so it reaches the same temperature as your tank. If the new water is much colder or warmer than the tank, the temperature shock can stress fish and trigger disease.

You can speed up temperature matching by placing the bucket near (not on) a heater or in a warm room. Use a thermometer to check both the bucket and the tank before you pour. The temperatures should be within 2 degrees Fahrenheit of each other.

Pour the new water back into the tank slowly

Pour the treated water into the tank slowly—over 2 to 3 minutes—so you do not create strong currents that stress the fish. If you have delicate fish like bettas or fry (baby fish), pour even more slowly or use a cup to add water a little at a time. Watch the fish as you pour; if they dart around or hide, you are adding water too fast.

Once the water level returns to where it started, you are done. The entire process usually takes 20 to 30 minutes for a small tank and up to an hour for a large one. Do this once a week at the same day and time so you build a routine and do not forget.

Clean the filter only when water flow slows down

The filter media (the sponge, cartridge, or floss inside the filter) traps waste and also hosts the bacteria that break down ammonia. Do not clean it during every water change. Instead, clean it only when water flow from the filter noticeably slows—usually every 2 to 4 weeks, depending on how many fish you have.

When you do clean the filter, rinse it in a bucket of old aquarium water that you removed during the water change. Never use tap water, because chlorine will kill the bacteria. Squeeze the media gently under water until the water runs clear, then put it back in the filter. This removes trapped waste while keeping the bacteria alive.

Test your water to track ammonia and nitrate

Buy a water test kit that measures ammonia, nitrite, and nitrate. A basic kit costs $20 to $35 and includes test tubes, reagent bottles, and a color chart. Test your water once a week, ideally before you do a water change, so you see the highest levels.

Ammonia and nitrite should always be zero. Nitrate should stay below 40 parts per million (ppm). If ammonia or nitrite is above zero, increase water changes to twice a week. If nitrate climbs above 40 ppm, do larger water changes (40 to 50 percent) or change water more often. If levels stay perfect for a month, you can test every two weeks instead.

Common mistakes that harm your tank

Do not use water that is too hot or too cold. Temperature swings of more than 5 degrees Fahrenheit stress fish and can trigger fin rot or ich (a parasitic disease). Do not forget the dechlorinator—chlorine kills the beneficial bacteria that keep ammonia from building up, and it damages fish gills. Do not scrub the gravel hard or remove the filter media during routine water changes; you will destroy the bacteria colony and cause an ammonia spike.

Do not change more than 50 percent of the water at once unless your tank has an ammonia or nitrite emergency. Large water changes remove too many bacteria and can cause the tank to cycle again, which means ammonia and nitrite spike for days or weeks. Do not skip water changes because the water looks clear—ammonia and nitrite are invisible, and fish can be dying from them while the water still looks fine.

Frequently Asked Questions

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

Bottled and distilled water lack minerals that fish need, so they are not ideal for regular water changes. Tap water with dechlorinator is best for most tanks. If your tap water is very hard or has high chloramine levels, you can mix bottled water with tap water, but use mostly tap water.

What if I do not have a gravel vacuum?

A basic siphon works fine—you just have to stir the gravel by hand with a stick or your hand before you siphon, so the waste floats up and comes out with the water. A gravel vacuum saves time and does both jobs at once, but it is not required.

How long can I leave a tank without a water change?

Most tanks can go two weeks without a water change if the filter is strong and the fish load is light, but one week is safer. Tanks with many fish or weak filtration need changes twice a week. If you go on vacation, do a large water change before you leave and ask someone to do one while you are gone.

Do I need to remove the fish during a water change?

No. Fish stay in the tank during a water change. Removing them stresses them more than the water change itself. The only exception is if you are doing a very large emergency water change (more than 75 percent) because of ammonia or nitrite poisoning.

Why is my water cloudy after a water change?

Cloudiness usually means the filter is stirring up waste or the gravel is releasing trapped sediment. It can also mean the bacteria are reproducing rapidly because you removed some of them. The cloudiness usually clears in 24 to 48 hours. If it lasts longer, your filter may need cleaning.