The basic process: drain, clean, and refill
Changing your aquarium water means removing a portion of the old water, cleaning the tank and decorations if needed, and adding fresh water back in. Most aquariums need a partial water change—removing 25 to 50 percent of the water—every one to two weeks. You do not need to empty the tank completely or remove the fish unless the water is severely contaminated or you are deep-cleaning the gravel.
The reason for regular water changes is straightforward: fish waste and uneaten food break down into ammonia and nitrate, which build up over time even with a filter running. A water change dilutes these compounds and keeps the tank chemistry stable. The process takes 20 to 45 minutes depending on tank size and how thorough you want to be.
Key Takeaways
- Drain 25 to 50 percent of the water using a siphon or aquarium pump, which removes waste from the bottom without disturbing the fish.
- Clean the glass walls and any decorations with an algae scraper or soft cloth while the water level is low, but do not rinse the filter media in tap water.
- Refill with dechlorinated water at the same temperature as the tank water to avoid shocking the fish.
- Let the water circulate through the filter for 10 to 15 minutes before feeding the fish again.
Draining the water safely
Use a siphon or a small aquarium pump to remove water. A siphon is a length of tubing that you fill with water, place one end in the tank, and let gravity pull water out into a bucket. An aquarium pump does the same job with a motor and is faster for large tanks. Both methods let you position the intake tube near the gravel where waste collects, so you remove debris along with the water.
Start the siphon by filling the tube with water from the tank itself—do not use your mouth, as tap water may contain chlorine. Once water is flowing, place the intake end on the tank bottom and let it run into a bucket. Move the tube slowly across the gravel to stir up settled waste and pull it out. Stop when you have removed the amount you planned—usually one-quarter to one-half of the tank volume.
If you have a very small tank (under 5 gallons), a siphon may be awkward. Instead, use a small cup or turkey baster to scoop out water from different areas of the tank, which takes longer but works just as well.
Cleaning the tank and decorations
While the water level is lower, wipe the glass walls with an algae scraper or a soft cloth to remove algae buildup. Scrape gently so you do not scratch acrylic tanks. If you have plastic or silk plants, decorations, or rocks, rinse them in the water you just removed from the tank—never use tap water, which can introduce chlorine or harm beneficial bacteria on the surfaces.
Do not clean the filter media (the sponge, cartridge, or floss inside the filter). Rinsing it in tap water kills the bacteria colonies that break down fish waste. If the filter is clogged and water flow is very slow, rinse the media gently in a bucket of the old tank water you removed. This preserves the bacteria while clearing debris.
Preparing and adding fresh water
Fill a bucket with tap water and let it sit for 24 hours, or use a water conditioner product to remove chlorine and chloramine when ready. Most municipal tap water contains these chemicals to kill bacteria for human use, but they harm fish and the beneficial bacteria in your filter. Water conditioners are inexpensive and available at any pet store—follow the bottle instructions for dosage based on the amount of water you are adding.
Before pouring the new water in, check its temperature with a thermometer. It should match the tank temperature within a few degrees. Cold water shocks fish and can cause stress or illness. If the new water is cooler, let it warm to room temperature, or mix it slowly into the tank over several minutes rather than pouring it all at once.
Pour the new water slowly into the tank, aiming for a decoration or the side wall rather than directly onto the gravel, which can cloud the water and disturb the fish. Fill until the water level returns to where it was before the change.
After the water change
Turn the filter back on if you turned it off during the change, and let it run for 10 to 15 minutes before feeding the fish. This gives the water time to circulate and stabilize. The fish may seem stressed or hide for a few minutes after a water change—this is normal and they will return to their usual behavior once the water settles.
If you notice the water becoming cloudy or murky within hours of a change, it usually means the filter is working through a spike in organic matter, or the new water was not dechlorinated properly. The cloudiness typically clears within 24 hours. If it persists or the fish show signs of distress (gasping, clamped fins, erratic swimming), do a smaller water change and check that your filter is clean and running.
Tools and supplies you will need
| Item | Purpose | Notes |
|---|---|---|
| Siphon or aquarium pump | Remove old water | Siphons are cheap and need no power; pumps are faster for large tanks |
| Bucket (5–10 gallons) | Catch removed water | Use one dedicated to the aquarium, not one used for cleaning chemicals |
| Algae scraper | Clean glass walls | Plastic scrapers for acrylic tanks; metal blades for glass |
| Water conditioner | Remove chlorine from tap water | Optional if you let water sit 24 hours, but much faster |
| Thermometer | Check water temperature | Stick-on or floating types both work |
Common mistakes to avoid
Do not change more than 50 percent of the water at once unless the tank is heavily contaminated. Large water changes remove beneficial bacteria and can cause ammonia spikes that harm fish. If the water is very dirty, do a 25 percent change and repeat it two or three days later rather than draining half the tank in one go.
Do not use soap, bleach, or household cleaners on anything that goes in the tank. Even tiny residues are toxic to fish. If you need to disinfect a decoration, use a 10 percent bleach solution (one part bleach to nine parts water), rinse it thoroughly under running water, and let it air-dry completely before returning it to the tank.
Do not forget to dechlorinate the new water. Chlorine evaporates slowly—leaving water out for 24 hours works, but a water conditioner is faster and more reliable. Many fish keepers keep a bucket of dechlorinated water sitting in a corner so it is always ready for the next change.
Frequently Asked Questions
How often should I change the water?
Most tanks need a 25 to 50 percent water change every one to two weeks. Smaller tanks (under 20 gallons) and tanks with more fish need changes more often—sometimes weekly. Tanks with heavy filtration and few fish can go longer. Watch the water clarity and smell; if it looks cloudy or smells stale, change it sooner.
Can I use bottled water instead of tap water?
Bottled water works but is expensive for regular changes. If your tap water is very hard or has high chlorine levels, bottled water is a reasonable option. Distilled water should not be used alone because it lacks minerals fish need, though mixing it with tap water is fine. Most tap water with a conditioner works just as well and costs far less.
What if I do not have a siphon?
A small cup, ladle, or turkey baster works for tanks under 10 gallons. For larger tanks, a siphon costs $5 to $15 and saves time. You can also buy a small submersible pump for $20 to $40 if you plan to do frequent large changes. Both are worth the investment if you keep fish long-term.
Do I need to remove the fish during a water change?
No. Fish can stay in the tank during a partial water change as long as you drain and refill slowly and keep the water temperature stable. Remove fish only if you are doing a complete tank cleaning, treating the tank for disease, or the water is so dirty that visibility is nearly zero.
Why does the water get cloudy after I change it?
Cloudiness usually means the filter is processing a temporary spike in organic matter, or the new water was not fully dechlorinated. It clears within 24 hours in most cases. If it persists, check that your filter is clean and running, and verify that you used a water conditioner on the new water.