A siphon uses gravity and water pressure to move water from your tank into a bucket without lifting the tank or using a pump

An aquarium siphon is a tube that drains water downward once you start the flow. You fill the tube with water, seal both ends, position one end in the tank and the other in a bucket on the floor, then release the lower end—gravity pulls the water out. The whole process takes about 30 seconds to set up and works on tanks of any size, from 10 gallons to 100 gallons. No electricity, no moving parts, no noise.

Most siphons come as a single rigid or flexible tube, sometimes with a gravel vacuum attachment on the intake end. The gravel vacuum is a wide tube that slides over the main siphon tube and stirs up debris as water flows through—this is the version most aquarium keepers use because it cleans the substrate at the same time you drain the tank.

Key Takeaways

  • Fill the siphon tube completely with water before you start, either by submerging it in the tank or using your mouth to draw water through.
  • Position the intake end (in the tank) higher than the output end (in the bucket) so gravity can pull the water downward continuously.
  • Once water starts flowing, you can move the intake end around the tank bottom to stir up debris and remove it from the water.
  • Stop the flow by pinching the tube, lifting the output end above the intake end, or pulling the intake end out of the water.
  • A siphon works best when the bucket sits on the floor and the tank sits on a stand or table at least 12 inches higher.

Fill the siphon tube with water before you begin

The siphon will not start on its own—you have to fill it with water first. Submerge the entire tube under the water in your tank, making sure no air bubbles are trapped inside. Tilt the tube as you push it down so air escapes from the top end. Once the tube is completely full and underwater, you are ready to move it into position.

If you are using a gravel vacuum, submerge the wide outer tube along with the inner siphon tube. You may need to push down gently on the gravel vacuum to keep it fully submerged while you fill it. Some people find it easier to fill a siphon by placing one end in the tank, sealing the other end with their thumb, and drawing water up through the tube with their mouth—this works, but submerging is faster and cleaner.

Position the intake and output ends correctly

The intake end (the end inside the tank) must stay underwater throughout the process. The output end (the end going into the bucket) must hang lower than the intake end—this is what makes gravity pull the water out. If your bucket sits on the floor and your tank sits on a stand, the height difference is usually enough. If both are at the same height, the siphon will slow down or stop.

Hold the intake end in one hand and let the output end dangle into the bucket. Keep the intake end submerged but do not push it to the bottom yet—you want to be able to move it around. Once water starts flowing steadily, you can lower the intake end toward the gravel and begin cleaning.

Start the siphon by releasing the sealed end

If you filled the tube by submerging it, keep one end sealed with your thumb as you lift the tube out of the tank. Move quickly to position the intake end back in the tank and the output end over the bucket. Release your thumb from the output end—water should flow when ready. If it does not, the output end may be too high, or an air bubble may be trapped in the tube. Push the intake end deeper into the tank or lower the output end and try again.

Once water flows, it will continue flowing as long as the intake end stays underwater and lower than the output end. You do not have to do anything else—gravity does the work. The flow will slow slightly as the water level in the tank drops, but it will keep going until the tank is nearly empty or you stop it.

Use the gravel vacuum to clean the substrate while draining

If your siphon has a gravel vacuum attachment, push the wide outer tube down into the gravel once the water is flowing. The vacuum will stir up debris—dead plant matter, uneaten food, fish waste—and pull it into the siphon tube along with the water. Move the vacuum slowly across the bottom of the tank, spending a few seconds in each spot. You will see debris swirl up into the tube and disappear into the bucket.

Do not jam the vacuum into the gravel or hold it in one spot too long. A gentle push-and-lift motion works best. If the flow slows down, the intake may be clogged with debris—lift the vacuum slightly and let the flow clear it, then continue. Most aquarium keepers clean the gravel in sections, working from one corner of the tank to the other.

Stop the siphon when you have drained enough water

You have three ways to stop the flow. The fastest is to pinch the tube with your fingers to block the water. The second is to lift the output end above the level of the intake end—gravity reverses and the water stops flowing. The third is to pull the intake end out of the water. Any of these will work, and you can restart the siphon by repeating the steps above.

Most tank cleanings remove 25 to 50 percent of the water, depending on how dirty the tank is. A heavily planted tank with light bioload may need only 20 percent removed every week. A tank with many fish and little plant life may need 40 to 50 percent removed. Start with 25 percent and see how the tank looks after a few days.

Troubleshooting common siphon problems

If water will not start flowing, check three things: the intake end must be underwater, the output end must be lower than the intake end, and the tube must be completely filled with water (no air pockets). If all three are correct and water still will not flow, the tube may have a crack or hole. Hold it up to the light and look for damage. A small hole will stop a siphon completely.

If the flow is very slow, the intake may be partially clogged with gravel or debris. Lift the intake end slightly to let water flow backward through the tube for a second, clearing the blockage. If you are using a gravel vacuum, make sure the intake holes on the vacuum are not covered by gravel. If the flow stops suddenly while you are cleaning, you may have pulled the intake end above the output end by accident—lower it back down and the flow will restart.

Frequently Asked Questions

Can I use a siphon on a tank without a stand?

Yes, but you need the output end (bucket) to be lower than the intake end (in the tank). If your tank sits on the floor, you can place the bucket on the floor next to it and tilt the tank slightly, or you can hold the intake end high in the tank and let the output tube run down the side. The height difference does not have to be large—even 6 inches of drop will keep water flowing.

Is it safe to use my mouth to start a siphon?

Yes, it is safe as long as the tank water is clean and you do not swallow any. However, submerging the tube in the tank first is faster and avoids the issue entirely. If you do use your mouth, draw gently and stop as soon as water reaches your lips—you do not need to pull hard.

What size siphon do I need for my tank?

Siphon tubes come in diameters from half an inch to over an inch. Larger tubes move more water per minute, which is useful for big tanks or when you are in a hurry. For tanks under 50 gallons, a half-inch or three-quarter-inch tube works fine. For larger tanks, a one-inch tube will drain faster and make cleaning easier. The tube length should be long enough to reach from your tank to a bucket on the floor—usually 4 to 6 feet.

How often should I use a siphon to clean my tank?

Most aquarium keepers siphon once a week during a partial water change. The frequency depends on how many fish you have, how much you feed them, and whether you have live plants. A lightly stocked planted tank may need cleaning every two weeks. A heavily stocked tank with no plants may need cleaning twice a week. Watch the water clarity and the amount of debris on the bottom to decide what works for your tank.

Can I siphon water from one tank into another tank?

Yes, this is a common way to move water between tanks or to fill a new tank with established water from an old one. Position the output end in the receiving tank instead of a bucket. Make sure both tanks are at a height where the intake end is higher than the output end, or the siphon will not work. This method is much faster than carrying buckets back and forth.