How to Build a Bell Siphon


The easiest way I have found to make and fit a bell siphon in my Aquaponic grow bed
One of the most fun and possibly frustrating parts of Aquaponics is the building and fitting of a Siphon to empty your grow bed once it has filled.
Ideally you want the water in the grow bed to fill to just below the surface of your grow media.

It then can automatically empty, to allow your grow bed to breathe, this is not essential as many people use constant flood grow beds, personally it seems logical to me to help with oxygenation of the water therefore I use siphons

I Believe that getting the water coming into your grow bed and then to have it siphon away is desirable for healthy plant growth. (some plants don’t do as well with their roots constantly covered by water.)

How to build a bell siphon.

Below I will describe how I build my siphons, which I have simplified as much as possible to give consistent results.
I will list all the parts needed and steps to take with straight forward simple instructions and process of fitting and the operation of the siphon.
There are many ways to build siphons and all of them work, this is just my way of doing it and I find it the easiest and you may also find it works for you.
Read on and learn how I Build a Siphon.

The things you need for your bell siphon
First as always you need to gather everything you need for your bell siphon
1 – Drill
2 – PVC Pipe cutter (optional)
3 – 32 mm hole saw
4 – 8 mm drill bit
5 – Saw (I used a circular power saw)
6 – 20 mm tank fitting Female
7 – 20 mm PVC Pipe
8 – 40 mm PVC pipe
9 – 40 mm pipe cap
10 – 90 or 100 mm PVC pipe
11 – 20 mm elbow
12 – 300 mm x 20 mm riser (threaded each end)
13 – 13 mm plastic tube (or piece of hose from garden hose)
14 – Appropriate safety equipment, use at all times.
15 – Approx. 200 mm deep Grow bed (grow beds can be any depth just get a riser that will be longer than your grow bed will be deep.)

Building your Aquaponic siphon
First find a spot that will not interfere with your supports for the grow bed and drill the 32 mm hole with your hole saw
Fit the 20 mm tank fitting into the hole and tighten to make water tight.


Bulkhead fitting


Screw the riser into the tank fitting



Mark the riser where you want your water height to be.
Remove the riser cut the riser with your PVC pipe cutter (knife, saw)
Screw the riser back into the tank fitting, then push the piece of 13 mm tube


Tube used for insert

into the riser and then screw the riser off cut into the underside side of your tank fitting. ( I used a piece of irrigation tube that I had laying around) the short piece of tube causes a funnelling effect that draws the air out of the siphon creating a vacuum (it is very important to have no air leaks or your siphon will not drain your grow bed.)

Cut your 40 PVC pipe a little longer than the riser inside the grow bed ( I have my housing pipe with cap total height of riser plus riser diameter  therefore if my riser is 180 mm above the grow bed base and is 20 mm in diameter my 40 mm housing with cap would be 200 mm )
In the base of the 40 mm pipe drill a hole right through both sides of the pipe making 2 holes (this allows the water to flow into the siphon)
at 90 degrees to the first 2 holes and higher up the pipe ( just above the first holes is best) drill right through once again this will be your air intake that will break the siphon.

You should now have a piece of pipe with four holes in it, with 2 holes being slightly higher than the other 2.
Place the 40 mm cap on the pipe.


Outer tube with cap

If you are worried about not enough water getting into the siphon go right ahead and add another 2 holes.
I used an 8 mm drill bit, 4 holes means 32 mm of inflow, since the riser is only 20 mm with a reducer in it, I feel that 4 holes is enough.

Take the 90 mm pipe measure and mark to just above the height of where your grow bed media will fill to.

Using the saw ( set the circular saw to a very shallow depth) make a row of cuts to just below your mark

Rotate the pipe a little and repeat the row of cuts making them just offset from the first row making sure that they don’t touch each other or are so close together that they weaken the guard.
Repeat this until you have created a length of pipe filled with slots. Once done, cut off the pipe at your mark. (this will be your media guard.)


Media Guard

Place the media Guard over the Siphon

I Added an elbow to the cut off piece of riser that is attached to the outward side of the tank, then pushed in a short piece of 20 mm PVC Pipe
I then added a T Piece. The T Piece is there to add other optional outlets. (sprayers for aeration etc.)
If your happy with the water just dropping back into your tank, then you don’t need to use it.
Here is a picture of my siphon in action

Siphon in Action

Siphon in Action

You now have a functioning siphon. All you need to do now is adjust the rate of the water entering your grow bed to enable the grow bed to empty. If the water flow into your tank is too fast your bed will empty but the siphon will not stop and there is a chance your grow bed will not fill.

The sizes I have used, work for me, with a 200 mm deep grow bed 1 metre x 1 metre with a fill and empty of approx. 4 times per hour
You can use different sizes to suit your needs, also the sizes were partly chosen for easy availability of parts, in my case all the parts were obtained from bunnings and I was able to build it without having to use size adapters which were harder to find.
Also no gluing is necessary, just so long as you push everything together firmly enough to make sure there are no air leaks, this makes it possible to dismantle and make adjustments.

How do I determine what pipe size to use for my grow-beds?
For starters since I use the metric system it is quite simple maths.
That and the fact that there are not a lot of different pipe sizes available,
and this shows how this not a science but really a best guess that works for you and what’s available.
I doubt whether the plants are going to grow any less effectively if you have a drain and fill time of 20 minutes instead of 15 minutes

First I simply measure the depth of my grow bed in this case, my beds varied from 200 to 230 mm (just because that’s the height of the first rail on the IBC )and bed size was 1m x 1m this is a volume of roughly 200 litres.

So I divide by 10, this is 20 to 23 so the size stand pipe I need is one that is available, which is 20mm and I start from there, then using the simplest way I know how to make sure the other pipes give you a working siphon, I double the pipe for each one.
For instance
Stand pipe is 20mm then bell housing would need to be 40mm and media guard would need to be at least 90mm(cant get 80mm pipe)

That means if my tank was 400 mm deep or thereabouts I would start with a 40mm standpipe. 90mm bell housing and whatever I could find that would stop the media from interfering with the setup possibly 150mm

If you don’t have 1m x 1m grow-beds then all you need to do is work out the volume in litres and then divide by 10 to find your stand pipe diameter.

I am well aware that this can’t be a suits all solution, but I have found as a general rule that this seems to work for me.
A lot of people will say my beds are too shallow at 200 mm and that I should have no less than 300 mm in which case I would use 25 mm stand pipe. (deeper beds give more filtration and are easier to configure siphons, also increases the variety of plants that can be successfully grown)

The things is, its not rocket science, close enough IS good enough, the objective is not to be lost in math but to have fish and plants that survive and can be eaten.
If you think my math is flawed by all means leave a comment just keep it civil, I have outlined what is currently working for me, my fish are alive and my plants are growing, as I make more tanks and grow-beds I may revise the math I use, if I do I shall let you all know.

Here are some tips I have found to help with the operation of Aquaponics Bell Siphons
Just some of the useful things that I have found while building Aquaponic bell siphons.
First one is that although if you are using 20 mm pipe then a 90 mm media guard is sufficient, you may find it more beneficial to use 100 mm as this will help when doing maintenance, as you can get you hand into 100 mm pipe a lot easier.

Another is that the riser you use once screwed into the tank fitting can bind over time and if you need to remove it to clear pipes can prove difficult, to solve this I made my riser slightly higher but cut a groove with a saw across the top, making a slot down the riser on each side. This meant that I could place a flat piece of metal into the grooves to aid in turning the riser to unscrew it.

Although I didn’t glue any of my pipes to make making changes to the plumbing easier, I found it beneficial to have some flashing tape available to wrap around any joints that persisted on leaking or was causing air to enter the pipes stopping siphons from working properly. You can use silicon to seal the outside of the pipe this can be easily removed and the pipes taken apart if needed.

If you have long runs (anything over 1.5 metres I would do this) from you grow bed outlet back to your fish tank, have your outlet drop its load into a larger diameter pipe which takes the water back to the fish tank. This help in preventing airlock or back pressure which can result in unreliable siphon operation

To get your siphon to “trigger” you need a constriction within your pipe to speed up the water flow in your pipe outlet.
You can do this by having a larger inlet (the Bell) which sits on top of a smaller diameter pipe.
In the example above I pushed a piece of hose into the pipe creating the same effect, which I find easier but if you are using larger pipe, it is more difficult to create the restriction needed. To do this I simply cut 2 pieces of the same diameter pipe approx 1 cm wide,  then I cut out a small section out of each, making them into a C shape, squeeze one and place it into the other forming a O, if you then squeeze them both you can fit them into the pipe to create your restriction.

5 thoughts on “How to Build a Bell Siphon

  1. Garry B

    Thanks. A good description AND metric measurements. It does my head in reading posts using inches etc.
    Tomorrow I’m off to Bunnings for the bits

  2. alan partelle

    hi i have just stated to get into aqaponics and found your article on bell syphon outstanding i have built the syphon and tomorrow will be starting on the grow bed and tank not sure what size just yet thinking of using plastic fish boxes to start with once i get more confident ill look at going a bit bigger do you have any suggestion on any certain growing medium ill keep you updated how go and i am with you hate inches give me metric lol thans again for the tips

    1. esckay Post author

      once you get started you will find it absorbing
      There is no “best” media people use all sorts of stuff the easiest to use is gravel as it is cheap, its better if you do use to use river rocks around 20mm is ok, too small and they clog easy they are easier on your hands
      If you want to go to the expense then you can purchase media volcanic rock is another that some use or you can get hydroton which is light.
      the main thing to be aware of is that the media you use needs to be PH neutral. a Quick test, if you place a few in a glass and add vinegar and it bubble its not suitable to use.

  3. Brayden

    I found this really helpful and I’ve built this today but is there any chance you could send a link to or post pictures of the finished product broken down?

    My siphon only drains to the bottom of the cap on the 40 mm pvc as it’s not stuck together and is there a requirement for the length of the 13 mm tube? Should it sit at the bottom or be long enough to be visible at the top?

    Thanks in advance and that’s for finally putting it in metric!!!

  4. esckay Post author

    HI Brayden sorry for taking so long to get back to you
    We have just purchased 160 acres and will be setting up an eco tourist business, and naturally a much bigger aquaponics setup, I have been very busy lately,

    The length of the 13mm doesn’t have to be of any particular length just so long as it reduces to “speedup” the water flow I use them straight from bunnings at the length they come in, approx 2 – 3 cm, there are all different diameters there.

    I push it down about a half a finger length.


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