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Oxygen is the most important water quality factor for healthy fish. We can provide oxygen through different forms or techniques, via water features or waterfalls, venturi or air pumps.

A venturi is one of the most efficient ways to oxygenate water and to ensure optimum water circulation in a pond. Some experts don't recommend venturis, stating that they can be inefficient devices because it forces the pump to push water through a restrictive venturi tube. This argument is certainly valid if one pump needs to return water to the pond through only this one device. Normally in fair sized ponds there are more than one return, all controlled through valves to regulate the flow needed at every point. 

Another argument is that venturis can often aerate so vigorously that they disturb the water surface to the extent that they prevent unhindered viewing of Koi. They also look quite impressive but tend only to aerate the upper layers of pond water. All these arguments are certainly true to an extent but the purpose of this article is mainly to assist the Koi keeper to make the most of the venturi should he/she wish to install one.

The last argument against the use of a venturi is the fact that it is actually a device that protrudes into the pond and may injure the fish. Nowadays, venturis are installed on the outside of the pond with the outlet flush with the inside of the pond wall, just like any other water return.


Venturis are simple and inexpensive devices installed in a pond and don’t require much maintenance. It is installed in line of one of the water returns, so no additional air pumps are required. It is also fairly simple to make one yourself. It works on the following principle:  As a fluid passes through a pipe that narrows or widens, the velocity and pressure of the fluid vary. As the pipe narrows, the fluid flows more quickly. Surprisingly, Bernoulli's Principle tells us that as the fluid flows more quickly through the narrow sections, the pressure actually decreases rather than increases! The increase in velocity through the injection chamber results in a decrease in pressure, thereby enabling air to be drawn through a suction port and entrained into the water stream.


The venturi has two major functions in a pond, as well as a “nice to have feature”. The first is to provide currents in the pond and the second is to add additional oxygen to the water. The combined water and air that is pushed into the pond below the water surface creates a stronger current than when the return flow consists of just water. Although the bubbles created by the venturi do ad some oxygen to the pond water, the greatest advantage is the current that it creates, bringing more water in contact with the surface where gas exchanges take place.

The nice to have feature that I referred to, is actually for me the most convenient and safe way of adding medication to a pond. The challenge we all face is to distribute the medication through the pond without creating “hot spots” of concentrated chemicals that may harm the fish. Many years ago I was advised to pour the medication all around the edges of the pond to prevent these “hot spots”. I tried my best to distribute it evenly around the pond, but as the Koi begin to associate you with food, they keep on following you to wherever you stand at the pond edge and naturally they come into contact with the concentrated chemical. With Malachite Green and also with Potassium Permanganate, one can clearly see that the clouds of chemicals take a few minutes to disperse evenly in a pond.

Now let me start at the beginning. During normal operation of the venturi, air is drawn through a tube that acts as a suction port. A venturi makes a very annoying gurgling sound and one way to prevent the noise is to push a piece of foam into the opening  of the suction port to muffle the sound, but still let some air through. Another way is to put a mall plastic container with a 3 cm hole drilled into the bottom, over the tube. The third way is to connect a length of aquarium air tubing to the suction port. This small tube gets rid of the noise, and is very handy to distribute chemicals slowly and evenly in the pond. Simply mix the correct dosage of medication with pond water in a 5 litre bucket, place the bucket next to the pond and drop the open end of the air tubing into the bucket. The venturi will slowly suck out the contents of the bucket while distributing it vigorously around the pond. This “nice to have” feature, but works like a charm!



Last Updated on Tuesday, 04 August 2009 13:45