Aquaponics - Short siphon

A bell siphon usually dumps all the water from an aquaponics grow bed. It does this by creating a siphon once the water gets to a certain level.

The cool thing about it, is that once triggered, it lifts water up and over a standpipe, and does this until the grow bed has drained. This gives us our flood and drain cycle in our grow bed.

One alternative to using a bell siphon, is to run your crow bed as constant flood. To do this all we have to do is remove the bell, and leave the water to reach the level of the standpipe, and circulate.

It seems constant flood has some advantages.

A constant flood system holds more water, so might be a bit more stable. Whenever anything changes in a system, having more water means that it tends to change more slowly. There is also probably a bit more real estate for the micro beasties to live in.

Most plants don't seem to mind so it looks like a thing worth doing on at least some grow beds if you have more than one.

I only have one, and its very stable, but my fish are getting bigger, so there might come a time when I'm running my system a bit closer to its fish holding capacity. With this in mind I'd like to maximise my system's ability to deal with the extra load.

One possible problem with constant flood, is that over time it might be more likely for areas to become stagnant. Constant flood pulls water from the top. If you are adding water to the top and taking it from the top, the water might ignore your desire to move through the system, and just take a short cut straight from the inlet to the outlet.

A system using a bell siphon drains in a fairly dramatic fashion as all the water is dumped, often a lot faster than it went in. This sucks a lot of air down into the media, and also creates a powerful surge throughout the system. The strong surge might help to distribute solids away from the water inlets, spreading them more evenly through the grow bed. A siphon pulls water from the bottom, and this might also aid in distributing nutrient evenly.

Constant flood = good
Flood and drain = good.

I hate the way the universe can do that sometimes. Make up your mind universe.

In an effort to greedily get the best of both worlds, I have changed my siphon a bit.

The small tube on the outside is an air breather pipe. It's purpose is to aid the siphon to stop in a decisive manner. If your siphon doesn't match your pump flow, there can be a condition where, at the end of the cycle when your siphon should stop, it continues to trickle out water at the same rate that the pump is pumping in. (see this post on calibrating a new bell siphon)

This means you system can be stuck on empty.

Plants and bacteria hate that.

If your plants and bacteria aren't happy, your fish are miserable.

When the water level gets down to the breather pipe, it suddenly sucks air and stops the siphon. Basically it just makes the siphon a bit more forgiving.

What this means, is that we can mount that breather pipe further up the bell, and when the water empties to that level, the siphon stops.

So now we can have a situation where the only 20% of the grow bed is drained, but it is still drained with a powerful surge, and it also still drains from the bottom. This may well provide a decent compromise.

I have a feeling plants will enjoy having their feet always wet, but their knees in flood and drain.

I'll let you know if anything terrible happens.


  1. Very informative, thanks for sharing.

  2. Hi there great article!!!

    I have the same siphon,same design only difference the external tube is smaller in diameter(standar pvc air tube for aquarium) my flood and drain system works but after a while stops working.The problem is that my air tube at some time holds water with little gaps of air and it stops working, i can solve this with bigger tube or something?

  3. I doubt the air tube is the problem. Air flows very easily through whatever size tube you have without any meaningfull restriction.

    I think the problem is more likely to be about the rate the water exits through the standpipe. Make sure the standpipe exits to clear air (ie no bends or anything other than exiting straight down to keep everything simple)

    also this from my post on calibrating a siphon ...

    - make sure your breather pipe is 2cm away from the bottom of your grow bed.

    - make sure the breather pipe is breathing fresh air when the water is low enough. Its possible, if the breather is a narrow enough pipe, and is hard against the siphon, to form a meniscus so that it sucks water even when it is above the level of the water. Check that the breather pipe is sucking air when it should be sucking air.

    hope that helps, but if ask again and I'll stick with it until it's solved.

    -120 things in 20 years


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