The basic steps for draining an aquarium

Draining an aquarium means removing water while keeping your fish alive and your equipment intact. The safest method is a siphon—a tube that uses gravity to pull water out—combined with a bucket or drain line. You'll remove 25 to 50 percent of the water for a routine cleaning, or drain the tank completely if you're moving, doing major repairs, or treating a serious problem.

The process takes 15 minutes to an hour depending on tank size. You'll need a siphon (or a submersible pump), a bucket or hose, a net, and containers for your fish if you're doing a full drain. The key is moving slowly enough that you don't stress the fish or stir up debris from the substrate.

Key Takeaways

  • A siphon is the simplest tool: insert one end into the tank, start the flow by mouth or by filling the tube with water, and let gravity do the work into a bucket or drain line below tank level.
  • For partial water changes, remove water while fish remain in the tank; for full drains, net the fish into a separate container filled with water from the tank first.
  • Turn off heaters and filters before draining, and never let the intake tube of a filter run dry or you may damage the motor.
  • Drain slowly and avoid stirring up the substrate so you don't cloud the water or stress the fish with sudden changes.
  • If you're draining to move or repair the tank, keep the fish in treated water (with dechlorinator if tap water is your source) at the same temperature as the original tank.

Preparing the tank and equipment before you start

Before you touch the siphon, turn off the heater and filter. A heater running in low water can overheat and crack, and a filter intake that runs dry can burn out the motor. If your filter has an intake tube, make sure it stays submerged the entire time you're draining, or stop draining before the water level drops below the intake.

If you have a canister filter or wet/dry filter, close the ball valves on the intake and return lines before draining. This prevents water from siphoning out of the filter media and makes refilling easier. For a hang-on-back filter, straightforward turn it off and let it sit—you'll restart it once the tank is refilled.

Gather your tools within arm's reach: the siphon, a bucket or hose, a net, and a timer or clock. If you're doing a full drain and keeping fish, fill a separate container (a large bucket, cooler, or spare tank) with water from the aquarium before you start draining. This water is already at the right temperature and has the right chemistry for the fish.

Using a siphon to drain water

A siphon is a tube, usually 3/8 to 1/2 inch in diameter, that you place in the tank with one end submerged and the other end in a bucket or drain line positioned lower than the tank. Water flows downhill through the tube by gravity once you start the flow.

To start a siphon, fill the entire tube with water first—do not use your mouth to suck water up, because aquarium water can carry parasites or bacteria. Instead, hold one end of the tube in the tank and fill it from a pitcher or faucet until water runs out the other end, then quickly move that end into the bucket below. Water will continue flowing as long as the outlet stays lower than the water level in the tank.

If your siphon has a squeeze bulb (a small rubber ball on the tube), submerge the entire tube in the tank, squeeze the bulb several times to fill the tube with water, then move the outlet end into the bucket. The bulb creates suction to start the flow without you having to use your mouth.

Adjust the flow by pinching the tube or using a valve if your siphon has one. Slow drainage is better than fast—it gives the substrate time to settle and prevents sudden changes in water chemistry that stress fish. A full 50-gallon tank typically takes 20 to 30 minutes to drain halfway.

Draining with fish still in the tank

For a partial water change (the most common reason to drain), your fish stay in the tank. Position the siphon intake away from the fish and plants—aim for the area above the substrate where debris collects. Move slowly and watch for fish that swim toward the intake; they are curious but can get sucked against the opening if the flow is too strong.

If a fish does get pulled toward the intake, pinch the tube when ready to stop the flow. You can wrap the intake end of the siphon with a pre-filter sponge (a foam sleeve sold at aquarium stores) to prevent this. The sponge lets water through but blocks fish.

Stop draining when you've removed the amount of water you planned—usually 25 to 50 percent for a routine cleaning. The fish will remain calm because the water level is still high enough and the temperature hasn't changed. You'll refill with fresh, dechlorinated water at the same temperature as the tank.

Draining the entire tank

A full drain is necessary when you're moving the tank, replacing the substrate, treating a serious disease, or doing major repairs. Before you start, net the fish into a separate container filled with water from the tank. Work quickly and gently—netting stresses fish, and they can jump out of the container.

Once the fish are out, drain the tank completely using the siphon. As the water level drops below the filter intake, turn off the filter if you haven't already. Continue draining until the tank is empty or nearly empty (a small amount of water at the bottom is fine and makes cleanup easier).

Remove decorations, plants, and substrate if you're cleaning or replacing them. If you're keeping the substrate, you can rinse it in a separate bucket using some of the aquarium water you drained—never use tap water alone, because it will remove beneficial bacteria. Keep the fish in their temporary container at room temperature, away from direct sunlight and loud noise, for as short a time as possible. Most fish can survive several hours in a covered bucket with an air stone or gentle air pump, but aim to refill and return them within 2 to 4 hours.

Refilling after a drain

Before you refill, make sure the tank is clean and all equipment is in place. If you drained completely, reinstall the substrate, decorations, and plants. Fill the tank slowly with fresh water that has been treated with dechlorinator if your tap water is chlorinated—most municipal water is. The dechlorinator removes chlorine and chloramine, which harm fish and beneficial bacteria.

Use water as close to the original tank temperature as possible. If the water is much colder or warmer, let it sit in the tank for 15 to 20 minutes before returning the fish, so the temperature equalizes. Turn the heater back on and let it stabilize the temperature before adding fish.

Once the tank is full and the temperature is stable, turn the filter back on. Wait 5 to 10 minutes for the filter to settle, then gently pour the fish back into the tank using a net or by floating their container in the tank for 10 minutes so they adjust to the new water temperature. Do not dump them in suddenly.

Common mistakes to avoid when draining

The most common mistake is draining too fast, which stirs up debris and causes sudden swings in water chemistry. A second mistake is forgetting to turn off the heater, which can overheat and crack if the water level drops. A third is letting the filter intake run dry, which damages the impeller (the spinning part inside the motor).

Do not use a standard garden hose to siphon unless it's brand new and has never carried pesticides or fertilizer—these chemicals are toxic to fish. Aquarium-specific siphons are inexpensive and safe. Do not drain into a sink if you have a septic system and the tank is heavily planted or has algae, because the organic matter can clog the system; drain into a bucket instead and dispose of the water in a toilet or outside.

Do not leave fish in a temporary container without aeration for more than a few hours. If you're doing a full drain that will take longer, use an air pump and air stone to keep oxygen levels up. Do not refill with water that is much warmer or colder than the tank—temperature shock can kill fish.

When to call a professional

You do not need a professional to drain a healthy aquarium. However, if the tank is cracked or leaking, if you suspect a serious disease and need to treat the water with medication, or if the tank is very large (over 100 gallons) and you're uncomfortable managing the weight and flow, a professional aquarist or your local aquarium store can advise you on the safest approach.

If you're moving and need to transport the tank and fish, an aquarium store can help you plan the move so the fish survive the journey. If the tank has been neglected and the water is very dirty or foul-smelling, a professional can help you do a safe restart without killing the fish.

Frequently Asked Questions

Can I drain an aquarium into my yard or garden?

Yes, if the tank has no medication or chemicals in it. Aquarium water is rich in nitrogen and phosphorus and is good for plants. However, if you have a septic system, avoid draining large amounts of heavily planted or algae-filled water into it, because the organic matter can clog the drain field. Drain into a bucket and pour it on plants or grass instead.

How long can fish survive outside the tank during a drain?

Most fish can survive 3 to 4 hours in a bucket or container with water from the tank, as long as the water stays cool and the container is covered to prevent jumping. If the drain will take longer, use an air pump with an air stone to add oxygen. Do not leave fish in direct sunlight or in a warm room.

What if my siphon won't start?

Make sure the entire tube is filled with water before you move the outlet end into the bucket. If the tube has air pockets, water won't flow. Refill the tube completely and try again. If you have a squeeze bulb siphon, submerge it fully and squeeze the bulb several times. If the siphon is kinked or blocked, straighten it or rinse it out with water.

Do I need to drain the tank if I just want to clean the filter?

No. You can clean the filter media without draining the tank. Turn off the filter, remove the media, and rinse it in a bucket of water from the tank (not tap water, which kills beneficial bacteria). Reinstall the media and turn the filter back on. This takes 10 minutes and does not disturb the fish.

Can I use a submersible pump instead of a siphon?

Yes. A small submersible pump (sold as a utility pump or fountain pump) can drain a tank faster than a siphon. Place the pump intake on the tank bottom, run the discharge hose into a bucket or drain line, and plug it in. A pump is useful for large tanks or if you have limited hand strength, but it costs more than a siphon and uses electricity.