Good aeration in an aquarium is not a result of good water surface “movement”. It is the result of “surface turbulence” or “bubble turbulence”. The term “turbulence” refers to chaotic rapid movement of water in several directions at once, like the water movement in a rapids. Other ways of describing “surface turbulence” are “breaking the surface tension”, “choppy waves” or “surface agitation”. The “bubble turbulence” of many small rising bubbles also works well.
We go into depth on the science behind aeration with turbulent flow below. This wordy section is only for real nerds like the author.
The Complicated Science
There is a science field called “rheology”. In this field there is a very important number called a “Reynold’s number”. If the flow of water at an interface with air is fast enough to exceed the Reynold’s number for water, then the flow is very chaotic and turbulent. This is the type of flow which give good aeration.
Choppy water or water where the surface tension has been “broken up” are turbulent flow. When the flow is slower it is “laminar flow” (sometimes called “lamellar”). Laminar flow gives a smooth water surface with much less aeration taking place. Smooth flow and small ripples are laminar flow. In this case there is no “gray area”. Water flow is either turbulent or laminar. There is no gradual change between turbulent and laminar. This difficult concept is actually quite simple:
The concept is easy to see on the surface of the water:
The laminar flow gives only slight aeration while the area of turbulent flow gives excellent aeration.
A great example of this turbulent flow and its effect on an aquarium is the following:
Notice how the surface of the water is choppy waves. This is turbulence and will give excellent aeration and crystal-clear water. Look at the fish six feet down this aquarium. It can be seen as clear as the one right next to the glass.
The turbulence at the air/water interface in an aquarium is very important for aeration. This interface is where the oxygen gas exchange takes place. The more the surface water turbulence the more exchange takes place. It is optimum to have water pumps and filtration returns situated to maximize the water’s surface turbulence. It is important to note that turbulence and speed are not synonymous here. You want high turbulence.
Small bubbles rising through the water each have a small area of turbulent flow associated with them. This turbulent flow aerates very well. So a lot of small bubbles rising though the water give very good aeration, contrary to popular myth. We cover that is the section on air stones.
9.3. Air Stones
What is important is turbulence. Both many rising bubbles and vigorous surface turbulence over a wide area gives good aeration. This turbulent flow is very important. Ben Ochart turned the canister return feeds in his aquariums down one night and turned off his circulation pumps to “give his fish a rest”. In the morning he had an aquarium full of dead fish.
It is obvious that a setup that creates a rapid turbulent flow of the surface down the length of the aquarium is optimum for aeration. But some fish hate this. In my experience small fish and sedate, slow-moving fish hate it. Don’t do it with tetras, discus or angelfish. But African cichlids and rainbowfish love it.
The ideal if the aquarium inhabitants don’t like high flow rates is to impinge several different pump flows against each other and against the walls of the aquarium. Don’t flow the flow down the length of the aquarium. Run the flows across the width of the aquarium, impinging against each other at 90 to 180-degree angles wherever possible. This will give much better surface turbulence without creating unwanted high currents in the aquarium.
If one is aerating with the output of a powerhead or the output from a canister or sump filter, the outlet should be set up to maximize the water turbulence at the surface of the aquarium. It should have a duckbill or fan nozzle on it with the fan pointing up at the surface about twenty degrees off the horizontal. The nozzle needs to be about two inches below the water’s surface. This creates an optimum amount of turbulence for good aeration.
A bladed fan circulation pump exit which has a horizontal discharge right at the surface of an aquarium can do a reasonable job of aeration as there is a lot of surface turbulence induced.
A powerhead or a circulation pump discharge which is horizontal two or more inches below the surface becomes much less of an aerator. This is also true for canister or sump filter outlets which simply flow their water into the aquarium through a pipe flowing out horizontally or vertically well below the surface of the water.
The water flow from hang-on-back filters is somewhat complicated. A “standard” HOB shoots the water directly down into the water, creating very little surface flow or aeration. The biowheel variation improves the aeration. Also, some HOBs have a lip which flows the water out horizontally right at the aquariums surface. This considerably increases the surface turbulence and the aeration.
Obviously if a sump filter is used all the above and more applies. The sump can have a trickle filter which gives excellent aeration. The sump can also have some air stones in it giving aeration. And of course a fluidized bed will give great aeration. And finally, the discharge into the aquarium can give good aeration if surface turbulence is created by the outlet.
Source: Aquariumscience.org – David Bogert