In this article, we will look in detail at the utility of the siphon in aquaponics: what it is for, how it works, how to resolve operational problems, etc.

What is a siphon used for?

A siphon allows you to create tides in the grow bed; this is its primary function. No siphon, no tide (note that there are different types of siphons, such as the bell siphon).

It allows for a massive supply of oxygen for plants via their roots, as well as for the bacteria. This tide therefore provides them with all the oxygen they need.

Another function of the siphon, or perhaps a side effect, is that it prevents the accumulation of organic matter: dead roots, leaves... Everything that could clog up your bed.

And finally, one very important thing to note is that the siphon is mechanical. You must therefore absolutely have a pump. Generally, this is already the case when dealing with an aquaponic system.

How does it work?

First, you must have installed your tank. Once you have your tank, we will place the siphon. You will have a tube that determines the maximum water height; this is generally the one at the top. Thus, the water will rise in the tank and then flow out through this tube. You will also have two 90-degree elbows, which will allow the water to return to the fish. Above this drain tube, the bell is placed. This is why it is called a bell siphon. It is thanks to this bell that we are able to obtain a siphon. We will then add a protective cylinder to prevent the medium from getting into the bell or the drain tube. Therefore, your clay pebbles, your volcanic rock, etc., must stay in the grow bed and not obstruct the siphon.

How does it happen?

The pump placed in the fish tank will bring in the water. Little by little, the grow bed will fill up and the water will rise. Once the water has reached the height of the drain pipe, it will flow down into this pipe, accelerating; this acceleration will create a vacuum. It is thanks to this vacuum that all the water in the tank will be sucked into the drain pipe. This is what we call a siphon.

Some useful tips

First of all, you should know that the sizing of the siphon is empirical in terms of the pipe width in relation to the size of your tank. In short, you will have to apply the rule of doubles—that is to say, the cylinder protecting the bell should have a diameter that is double that of the bell and should also be able to let your hand pass through (this is important for maintenance). The diameter of the bell itself will be double the diameter of the drain pipe. So, if you have a pipe that is 3.2 cm, that will give us 6.4 for the bell and 12.8 for the protective cylinder.

Secondly, since it is empirical, you will set up your system and then test it empty, without putting in any medium or plants. Once the siphon starts and stops, you can add your medium—your clay pebbles, your volcanic rock—and do another test before adding the plants.

Thirdly, for the part draining the water into the fish tank, you will need two 90-degree angles. This will allow you to properly re-prime your siphon, which is very important. You will need to adjust the flow rate of your pump so that there are about three to four tides per hour; that is, every hour, your water should have risen three or four times and fallen three or four times.

Fourthly, you must ensure you can control the flow of incoming water, and for this, it is highly recommended to add a valve. It is often easier to have a valve at the entrance to the grow bed rather than having to modify the flow rate directly on the pump.

And finally, since aquaponics is a living system, you will inevitably have roots that will clog your siphon at some point. You should therefore check it regularly by putting your hand inside the protective cylinder.

The most frequently encountered problems with a siphon and the solutions

There are two types of problems:

1. The first is that the siphon does not start,
2. The second, the siphon starts but does not stop.

Why does the siphon not start?

We say that the siphon does not start when the water rises in the tank, reaches the top of the drain pipe, flows out, but is not sucked. It is as if you did not have a bell. The water level remains constant and, obviously, there is no tide. The first thing to do in this case is to check that nothing is obstructing the bottom of the bell and the bell protection.

Indeed, if clay pebbles or sand have gotten in there, or if your base is not perfectly flat, you will have issues. You must also check the seal of the bell, particularly the top part. Often, these are DIY systems, so there is the main tube and a cap on top. And if it is not airtight, the bell cannot perform its function.

Finally, if the water flow is too low—meaning that if the tide rises very slowly when it reaches the top of the drain pipe—it will drain away slowly and will not create acceleration. The siphon will therefore not engage.

Second problem: the siphon starts but does not stop

This means that all the water in the tank is sucked out, but the siphon does not break, and the water stays at the very bottom. This type of problem is more annoying than the first because your roots that were in the tide end up exposed to the air.

Three reasons may be the cause:

1. Check that the drain in the fish tank is not submerged in water. We must not forget that we need to regularly add water to our aquaponics tank because the plants transpire. Therefore, at some point, you may have added water without paying attention, and the last 90-degree elbow of the drain pipe stayed submerged. In that case, the siphon will not stop.
   
   
2. The second thing that can cause you problems is the configuration of the drain. Let's take, for example, that you see there is a first 90-degree bend, a tube, and a second 90-degree bend depending on the length of the pipe according to the dimensions of your tank. You may have issues with the siphon not stopping.
   
   
3. And finally, the last cause could be that the water flow rate is too high. The siphon triggers, the tank empties, but then the pump brings in so much water relative to the siphon's drainage speed that air cannot get in, and the siphon does not break. We end up with a siphon that doesn't stop and water that stays at the bottom. In this case, we will manually re-prime the siphon by lifting the bell—that is, we put our hand inside the protective cylinder and then lift the bell. We will turn it a little on itself, put it back at the bottom, and try to solve the problem by looking at the causes mentioned above.