Start with a partial water change, not a full drain

The most common mistake is emptying the tank completely and scrubbing it clean. This kills the beneficial bacteria living in your gravel and filter that keep ammonia and nitrite from poisoning your fish. Instead, remove about 25 percent of the water, clean what you can see, and refill with fresh water at the same temperature as what you removed.

A partial water change takes 20 to 30 minutes and should happen once a week for a tank with fish. If your tank is heavily stocked or you have fish that produce a lot of waste—like goldfish or plecos—do a 25 percent change twice a week instead. The goal is to remove dissolved waste without disrupting the bacteria colony that does the real work of keeping your tank safe.

Key Takeaways

  • Remove 25 percent of the water weekly; never drain and refill the entire tank, as this kills the bacteria that protect your fish from ammonia poisoning.
  • Use a gravel vacuum to siphon debris from the bottom while you drain water, which removes waste without disturbing the gravel itself.
  • Clean the filter media in old tank water you just removed, never in tap water, because chlorine and temperature shock will kill the bacteria you need.
  • Wipe the glass and decorations with a soft cloth or algae scraper; avoid soap and harsh chemicals that leave residue toxic to fish.
  • Refill with dechlorinated water at the same temperature as the water you removed to prevent stress on your fish.

Use a gravel vacuum to remove waste from the bottom

A gravel vacuum is a tube with a wider chamber at one end and a siphon hose at the other. You push the tube into the gravel, and water and debris flow out through the hose into a bucket. This removes uneaten food, fish waste, and dead plant matter that would otherwise break down into ammonia—the main toxin that kills fish in neglected tanks.

Start at one corner of the tank and work slowly across the bottom. Push the tube down about 2 inches into the gravel, then pull it back up slightly and let the suction do the work. You should see debris being pulled into the tube while the gravel itself mostly stays in place. Spend 10 to 15 minutes on this step; it is the single most important part of keeping your water clean between full filter cleanings.

Clean the filter in old tank water, not tap water

Your filter cartridge or sponge is covered in beneficial bacteria. If you rinse it under tap water, the chlorine kills those bacteria and the temperature shock stresses them further. Instead, take the water you just removed from the tank and use it to rinse the filter media gently in a bucket.

Squeeze the filter media in the old tank water a few times until the water runs mostly clear. Do not scrub it hard or wring it out—you are just removing the loose debris, not sterilizing it. If your filter has a mechanical pre-filter (a coarse sponge or floss layer), that one can be rinsed more aggressively because it traps larger particles and does not house as many bacteria. The biological media underneath should always be treated gently.

Wipe glass and decorations without chemicals

Algae buildup on the glass is cosmetic, not a health problem, but it does block your view of your fish. Use a soft cloth, a plastic algae scraper, or a magnetic algae cleaner to remove it. Never use soap, vinegar, or window cleaner—even small amounts left behind are toxic to fish.

For decorations and plants, a soft brush or cloth works well. If something has stubborn algae or brown buildup, soak it in old tank water for a few minutes, then scrub gently. If you need to remove it from the tank, do so during the water change so the tank is not left empty. Avoid bleach or any chemical treatment unless you are treating a disease outbreak, and even then, follow the medication instructions exactly.

Refill with water at the right temperature

Cold water shocking your fish causes stress and can trigger disease. Before you refill, measure the temperature of the water remaining in the tank with a thermometer. Fill a bucket with fresh water and let it sit until it reaches that same temperature, or use a heater to warm it.

If you are using tap water, treat it with a dechlorinator first—most come as a liquid or powder that you add to the bucket before pouring. Chlorine in tap water damages fish gills and kills beneficial bacteria. Pour the water in slowly, aiming at a decoration or your hand to break the flow so it does not stir up the gravel and cloud the tank. The whole refill should take 5 to 10 minutes.

Clean the glass walls during every water change

While the water level is lower, it is easier to reach the upper corners and edges of the tank. Use a soft cloth or plastic scraper to wipe down the inside of the glass. Work from top to bottom so debris falls into the water below, where the gravel vacuum will catch it on your next change.

If you have a magnetic algae cleaner, one pad goes inside the tank and one outside, and they attract to each other through the glass. Slide them together to scrape off algae without your hands getting wet. These work well for routine maintenance but may not remove stubborn brown diatom algae or hard water stains—for those, a plastic scraper and a little patience work better.

Establish a cleaning schedule that matches your tank

A 20-gallon tank with a few small fish and a good filter can go a week between water changes. A 10-gallon tank or one with large fish like goldfish needs a change twice a week. A heavily planted tank with few fish may only need a change every two weeks because plants consume some of the waste.

The real indicator is your water test results. If ammonia or nitrite ever show up on a test kit, you are not changing water often enough or your filter is not large enough. Most beginners underestimate how much waste their fish produce, so start with weekly 25 percent changes and adjust only if your water tests stay at zero ammonia and zero nitrite for several weeks in a row.

Frequently Asked Questions

Can I use tap water straight from the faucet?

Not without treatment. Tap water contains chlorine and sometimes chloramine, which damage fish gills and kill the bacteria in your filter. A dechlorinator removes these chemicals in seconds. You can buy liquid or powder dechlorinator at any aquarium store for a few dollars, and a bottle lasts months.

What if I see white cloudy water after a water change?

This is usually a bacterial bloom caused by temperature shock or a sudden change in water chemistry. It clears on its own in a few days. To prevent it, match the temperature of new water to the tank water before adding it, and add new water slowly rather than pouring it in all at once.

How often should I replace the filter cartridge?

Most cartridges last 4 to 6 weeks before they clog and restrict water flow. You will notice the water moving more slowly through the filter. Replace it, but rinse the old one in tank water first to seed the new cartridge with bacteria. Some filters use sponge media that can be rinsed and reused indefinitely.

Is it okay to do a 50 percent water change instead of 25 percent?

Yes, if your tank is overstocked or you missed a week of changes. Larger changes remove more waste but also remove more of the beneficial bacteria and can stress fish if the new water is not the same temperature. For routine maintenance, 25 percent weekly is gentler and works just as well.

What should I do if my fish are gasping at the surface?

This usually means low oxygen or high ammonia. Do a 25 percent water change when ready, then test your water for ammonia. If ammonia is above zero, increase your water change schedule to twice weekly and check that your filter is running and not clogged. If the problem continues, your tank may be overstocked for its size.