Eheim diffusor.

With this tank I am using an Eheim diffusor as filter outlet, which provides excellent aeration, as well as having a number of other benefits. An Eheim diffusor is just a little piece of plastic that sells for around $10, depending somewhat on the size of hose you want to use it with. You simply push it in the end of the hose of your filter outlet, where usually for example a spray bar would go. No airpump or other device is used to drive it. In the following I will try to explain how it works and what it does. As an aside, the spellings diffusor and diffuser seem to be used at random throughout the web, so when you look for more info on this device, make sure to search for both spellings! When I wrote this page, Eheim called the thing diffusor, so that's what I am using.

Venturi effect

An Eheim diffusor relies for it's functioning on the Venturi effect. The Wikipedia article you can read when you click on the link will explain to you in detail what that is. In a nutshell, when water flows through a thinner section of pipe, it will flow faster in that section than in the rest of the pipe, and the laws of physics dictate that the pressure in this section of pipe has to be lower. My background is in chemistry, not physics, so please don't ask me why that is and let's just take it for granted - or read the Wikipedia article. Of course, if you buy an Eheim diffusor, you can clearly see with your own eyes that this is the case, since a little airline tube plumbed onto the thinner section of pipe sucks air from the outside into the water stream, and this air is injected in very small bubbles into the tank. If you stick the end of this little airline tube into a glass of water, the water will be pumped out of the glass into your tank. This isn't very useful, of course, but it shows how a venturi valve can be used as a pump. In fact, many people take advantage of this when they use a Python aquarium maintenance system to remove water from their tank. A Python is a bigger venturi valve than an Eheim diffuser, in which the water flow from your faucet is send through a thinner section of pipe, and the resulting reduced pressure is used to drain your tank. Similar pumps to a Python are used in chemistry research labs to create a mild vacuum, and I bet that's how the person who developed the Python came up with the idea! Anyhow, an Eheim diffusor does not use the Venturi effect to pump water, it simply sucks air into the outlet stream of your filter, and using and Eheim diffuser instead of for example a spray bar has several advantages.

An Eheim diffusor provides a powerful current, but can easily be hidden behind a rock or piece of driftwood.

Advantages of Eheim diffusor compared to other filter outlets, such as a spraybar.

Better aeration.

A belief commonly expressed in online aquaristic forums is that an airstone in a tank contributes to aeration of the water mostly because it increases the surface area of the tank due to rippling, while the aeration provided by the bubbles themselves in negligible. The truth in this is that aeration is increased by increasing the surface area on which air and water can interact, and oxygen can get dissolved in the water. If there is a lot of rippling, and just a few big air bubbles from a pump causing it, then the increase in surface area due to rippling will be large compared to the surface area of the bubbles themselves, meaning the contribution to aeration by the bubbles themselves will be small.

An Eheim diffusor brings a very large number of very small air bubbles far below the water surface. I have no mathematical model to prove this, but my feeling is that in this case of many small bubbles the surface area of all the bubbles combined could be in the order of magnitude of the increase in surface area caused by rippling, if not larger. This of course would mean you get better aeration than by rippling alone. As an aside, it would make a cool science project for somebody to determine this experimentally, and it shouldn't be too hard to do!

Less noise

A submerged spraybar can contribute to aeration only through rippling, and so is clearly inferior to a diffusor. A spray bar mounted over the tank might be able to create a similar amount of bubbles, although I feel they are larger and hence have a smaller total surface area. In additon, a spray bar mounted above the water surface will make considerable noise when the water coming out of it hits the tank water. A diffusor can be mounted several inches below the water surface, and the water above it will very effectively muffle any sound coming from the diffusor. Thus, a diffusor is virtually silent with the only faint noise coming from bubbles reaching the tank surface. These bubbles are very small and thus create minimal noise.

More directed flow

In my 125G Tropheus tank I chose to filter with an Eheim 2260 canister filter, and wanted neither additional power heads nor air pumps in that tank. This caused me to forgo the use of an UGJ system, and I had to rely solely on the strong current created by the powerful filter to drive debris into the filter intake. This is facilitated enormously by the use of an Eheim diffusor, which creates a very strong current in the tank. The diffusor has a directed, jet-like outlet similar to a powerhead.

The diagram shows the flow in my 125G Tropheus tank, which is pictured in the photo.

The black dot is the filter intake. As you can see, the main water flow is basically circular in the area to the right of the huge piece of driftwood (black elliptical shape in the diagram), but enough water flows around the left hand side of the wood to take debris into the filter intake. The main dead spot in the tank is in the middle of the empty area on the right, but fish seem to take enough interest in that debris to swipe it back into the water flow.

Apart from helping to get debris into the filter, a strong current in a tank is appreciated by many fish, such as tropheus, mbuna, and plecos, to mention just a few. A strong current can also be helpful in aggression control - fish that are busy negotiating the current can not pursue and terrorize weaker fish as effectively as they could in a calm tank. Vinny Kutty reported at the OCA Extravaganza 2008 about significant successes in breeding Pike cichlids using strong currents in the tank for aggression control. However, there are also fish that do not like strong currents, such as discus, angelfish and frontosa, to give just a few examples.

In summary, an Eheim diffusor provides a lot of benefits for a $10 device. Try it, I am sure you'll like it!