What is a Bell Siphon and Why We Use it in Aquaponics

A bell siphon is an essential component of a media based aquaponics system. It is responsible for regulating the entire system's water flow and efficiently helping move the water from the media bed into the fish tank. If you're planning on setting up your media based aquaponics system, it is important that you know the function of a bell siphon to be successful. This article will explain what a bell siphon is and why we use it in aquaponics.

Aquaponics Grow Bed

What is a Bell Siphon?

A bell siphon is a simple yet effective device used in media based aquaponics systems to regulate water levels automatically without human intervention and provide a natural filtration system. The device consists of a few basic components that work together to create a siphon that allows water to flow in and out of the grow bed at the correct intervals. The bell siphon's purpose is to maintain a proper water level in the grow bed, provide a natural filtration system, and ensure the health of both the plants and fish in the aquaponics system.

How Does the Bell Siphon Works?

Bell siphons use a mechanism that leverages the forces of pressure and gravity. As your grow bed fills up with water and reaches the top of the standpipe located inside the siphon pipe, the water will flow through the reduce placed at the top of the standpipe and out through the standpipe into the fish tank at low pressure.

As water drains slowly out of the standpipe, the water builds up inside the bell and pushes the air out through the standpipe. The pressure inside the bell falls as a result.

The resulting low pressure in the bell will lead to a pressure difference between the bell and the atmosphere, making the siphon fire. The siphon pushes out and dumps the water rapidly through the standpipe at a higher pressure until the water level reaches the grow bed base.

As the water level approaches the siphon pipe base, air will enter the bell through the holes at the bottom of the siphon pipe, relieving the pressure difference between the bell and the atmosphere, which causes the siphon to break and halt the drain of the water.

As the water pump continues to pump, the water will keep pouring into the grow bed, and once the level reaches the top of the standpipe, the siphon will be triggered again, and the flood and drain cycle will repeat until the pump is stopped. The process of filling and draining continues, creating a natural filtration system that provides the plants with nutrients while keeping the water clean and healthy for the fish.

Benefits of Using Bell Siphon 

  1. It helps maintains a proper water level in the grow bed. The siphon allows water to flow in and out of the grow bed at the right intervals, preventing overflooding and drowning of the plant's roots.
  2. It ensures the plants receive nutrients and oxygen from the water. A bell siphon promotes healthy plant growth by maintaining a proper water level and minimizes the risk of fish suffering from poor water quality.
  3. A bell siphon creates a natural filtration system. As water flows through the grow bed, the plants and beneficial bacteria break down the fish waste into usable nutrients for the plants. This process helps to keep the water clean and healthy for the fish, reducing the need for artificial filtration systems. 
  4. A bell siphon also promotes water oxygenation in an aquaponics system. As water flows through the grow bed, it comes into contact with the air, allowing for gas exchange. This process helps ensure that the water is oxygen-rich, essential for healthy plant and fish growth. 
  5. The bell siphon process works automatically. Once the bell siphon is working, you don't need a timer, additional drainage equipment, or manual draining to use the bell siphon in your system. The continuous cycle prevents extra work on your part.
  6. Bell siphons are easy to maintain and simple to use.
  7. Bell siphons are also easy to set up and require no electricity to function, which enables your aquaponic system to work efficiently.

Factors to Consider Before Installing Bell Siphon

Before installing a bell siphon in your aquaponics system, you will need you need to consider these few important factors:

1. Bell Siphon Placement Consideration

Before you drill a hole in your grow bed to install your bell siphon and piping, consider where you want to put it. The preferred placement is in the center, corner, or farther from where the water enters the bed.

2. Bell Siphon Size Consideration

One of the questions in using a bell siphon is, what size will you use? The rule of thumb is that your main siphon components should equal or be slightly greater than the water inflow components.  

If you have a high volume of water entering your grow bed through a 40 mm pipe, but your standpipe and outflow are only 20 mm. Your 20 mm could not cope with the volume of water, and your grow bed would overflow. But if you reverse the situation and use the 40 mm outflow, it will easily handle the amount of water the 20 mm pipe could put into your grow bed. Your grow bed will not overflow. 

So it is essential to consider the amount of water flowing to your grow bed, your pump capacity and how fast the water will flow, and how many flood and drain cycles you will need to do in an hour in choosing the size of your bell siphon. 

Bell Siphon

Components of a Bell Siphon

The components of a bell siphon are simple and available at most local hardware stores. The media guard can be drilled or cut, depending on your preference. These are the components of a bell siphon. 

1. Bell Cap: When water overflows into the standpipe, low pressure will eventually build up under the bell cap. This will force out the water at a higher rate until the air enters the bell.

2. Reducer:This is placed on top of the standpipe, which helps the siphon discharge pipe create a smooth transition; a 2:1 ratio in a reducer is optimum. (if the standpipe is 1 inch long, a reducer should be 2 inches). 

3. Siphon Pipe: The siphon pipe should have a diameter twice as big as the reducer. Bell siphon pipe creates a barrier between the siphon and the atmosphere and prevents air from entering. It also allows the siphon to build up a negative pressure that creates a vacuum that keeps the water flowing in the siphon.

4. Stand Pipe: This is placed inside the siphon pipe and acts as the discharge pipe. Standpipe also regulates the maximum water level in the grow bed. Water flows out through the standpipe once it reaches the top of the standpipe. It also connects the bulkhead to the reducer.

5. Media Guard: The media guard prevents the growing media in the grow bed from clogging the standpipe and bell siphon.

6. Bulkhead: Bulkhead holds the standpipe in the grow bed and allows the water to drain through the standpipe without leaking back to the siphon.

7. Outlet Pipe: Also called a drainpipe, this extends from the bottom of the bulkhead into the fish tank. 

Putting the Bell Siphon Together

1. First, drill a hole in your grow bed, put an uniseal in the hole, and then place the standpipe. Ensure that you have the correct size drill bit for your PVC fittings (bulkhead).

2. Push the standpipe through the top of the uniseal until the standpipe's top is at the height of the bottom of the bell cap on the bell siphon.

3. Connect the drain to the fish tank using a 90-degree elbow into the bottom of the standpipe below the grow bed.

4. Position your grow media around the standpipe. Fill up your grow bed with grow media to the height of the top part of the standpipe. To check if your grow media are high enough, fill your grow bed with water; These will expose low spots.

5. Place the fish tank under your grow bed and fill up the tank with water once you have ensured that your grow bed has siphoned all the rinse water.

6. Place the pump and pipes in the fish tank. Make sure it is stable.

7. Turn on the pump to make sure your siphon works properly.

8. Once you have completed the setup and cycled your system, you can add your fish and plant your crops.

Troubleshooting common Bell Siphon Issues

    If your bell siphon is not working correctly, there may be several reasons. The most common issues include:

    1. The bell siphon is not level or centered.

    A bell siphon must be level and centered in the grow bed. If it is not, the water may not drain properly, causing flooding or inadequate drainage. Check the positioning of the bell siphon and adjust it as necessary.

    2. The vent tube is clogged or not positioned correctly.

    The vent tube allows air to enter the bell siphon, breaking the siphon and draining the water. If the vent tube is clogged or incorrectly positioned, air cannot enter the bell siphon, and the water will not drain properly. Check the positioning and condition of the vent tube and adjust it as necessary.

    3. The overflow outlet is not installed correctly.

    An overflow outlet allows excess water to flow out of the grow bed. If not installed correctly, water may not drain properly, causing flooding or inadequate drainage. Check the positioning and installation of the overflow outlet and adjust it as necessary.

    4. The height of the bell siphon is not adjusted correctly.

    The height of the bell siphon determines the water level in the grow bed. If not adjusted correctly, the water level may be too high or too low, causing flooding or inadequate drainage. Adjust the height of the bell siphon by raising or lowering it as necessary.

    To troubleshoot these issues:

    1. Check the positioning and installation of the bell siphon, vent tube, and overflow outlet.
    2. Make sure the height of the bell siphon is adjusted correctly.
    3. If necessary, adjust the components and test the system again.


    Using a bell siphon in your media-based aquaponics system is an excellent way to ensure that your system runs smoothly. Bell siphon works automatically and keeps the water in your system moves efficiently. By understanding what a bell siphon is, its components, and how it works, aquaponics enthusiasts can choose the appropriate type of bell siphon for their system and install it using the necessary tools and materials. Bell Siphons are a sustainable and cost-effective solution to aquaponics systems, and they are essential for those seeking a successful and healthy aquaponics setup.

    Thank you for reading our article. If you find our articles helpful, read our "Guide on Choosing the Right Aquaponics Grow Media." 

    1 Response

    martin picard

    martin picard

    July 27, 2021

    A very helpful article. What I believe is badly needed is a sketch or animated gif showing how it works. Thanks very much.

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