9.1. Aeration in Depth

Aeration is creating a water/air interface in the aquarium where oxygen and carbon dioxide exchange can take place. Fish breathe water. They need to get their oxygen from air dissolved into the water. So, this exchange is important.

For good aeration, one needs to have a chaotic turbulent flow of water at the interface with the air. Terms used to describe this turbulent flow are “surface agitation”, “choppy waves” or “breaking the surface tension”. Lots of bubbles rising through the water also give turbulent flow and excellent aeration (confirmed by testing and contrary to popular mythology) So good aeration can come from EITHER a lot of surface agitation or a lot of bubbles from a DECENT SIZED air pump and airstone combination.

Signs of poor aeration in the fish depend on the currents in the aquarium:

• In an aquarium with little current, fish will hang motionless at the top of the water close to the surface with their gills rapidly moving. They will also “pipe”, taking in the water at the surface of the aquarium where the water has more oxygen dissolved in it.
• In an aquarium with a wavemaker and/or lots of current the fish will swim in the current with their mouths gaping, allowing the current to flow water rapidly over their gills.

There are several YouTube videos that claim aeration isn’t important. They claim that “osmosis” and “diffusion” with no surface agitation and no bubblers are quite sufficient for the fish. These well-meaning folks invariably have low-tech planted aquariums with only a few fish. If their aquariums were even moderately stocked with fish, they would have a lot of dead fish. In any aquarium with decent amounts of fish, aeration is needed.

The Importance of “Turbulent Flow” to Aeration

One way to get good aeration is by having the surface of the aquarium be water which has rough turbulent “choppy” appearance to it. This can be the discharge from a filter outlet or the flow from an in-aquarium pump (a “wavemaker” or a “powerhead”). These are so-called “turbulent flow” areas which do an excellent job of aeration.

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.

The other way to get excellent aeration is through the use of airstones. Note that airstones do an excellent job of aeration, AS LONG AS THE AIR PUMP IS LARGE ENOUGH TO PUT OUT HUNDREDS OF BUBBLES PER SECOND. The small air pumps sold for small amounts in some stores or over the internet or supplied with some aquariums in kits are insufficient to do much aeration.

A table is instructive. Note that the variation from very good to very bad is huge when it comes to aeration, so an index of 100 is used. 100 is very good, 5 is very bad. The index is a VERY rough approximation of either the turbulent surface area or the calculated bubble surface area of each type of aeration. A ten-inch by ten-inch surface area turbulence will have a score of 100. Because of Brownian movement even a very still water surface will be doing some degree of exchange,

VERY ROUGHLY each 100 points that one has cumulatively on an aquarium can support a pound of fish. Now obviously each item will have large variations simply due to the size of each item. A five-gallon wet/dry filter will have only one-fifth the aeration capacity of a twenty-five-gallon wet dry filter.

Note that options 2 through 5 have turbulent flow water/air interfaces and have much higher ratings than the laminar flow of options 6 through 12.

Aeration in a Dry Climate

Note that, in a dry climate, any option that includes an air pump and an air stone can pump a lot of dry air through the aquarium. When this dry air picks up the moisture from the aquarium it will carry it out of the aquarium. This can give a significant problem with evaporation. Obviously the drier the climate one is in the worse the problem will be. This will also be a problem with air pump-operated K1 fluidized beds. In dry climates, it is best to do the aeration with choppy waves, not air stones and air pumps.

Why is Aeration Important

Aeration is important not only in that it directly helps a fish by letting it “breathe” easier, but also in that it indirectly helps the fish by cleaning up the water. Clean, bacteria-free, crystal clear water is THE KEY to good fish health.

What happens is that all the living things in an aquarium need oxygen to oxidize organic compounds and produce energy in order to live. Thus bacteria and most other tiny organisms might reproduce twice as fast at an oxygen content of 8 ppm as they reproduce at an oxygen content of 7 ppm. So lets say some excess food releases dissolved organic compounds over a span of three days.

The dissolve organic compounds feed bacteria and the bacteria reproduce. Bacteria in the water column is bad for fish. But bacteria are also killed and eaten by many little creatures in an aquarium so they have finite lives. The faster the bacteria reproduce the faster they use up the dissolved organic compounds released by the food. So the bacteria stop reproducing at half the time at an oxygen content of 8 ppm as opposed to an oxygen content of 7 ppm. So there is half the bacteria in the water at 8 ppm than there is at 7 ppm.

This is of course just a huge generalization and there are literally hundreds of variables here. But it is clear that the more aeration one can get into an aquarium the healthier the fish will be.

Aeration in More Depth

One note on aeration. Using aerosols and chemicals around an aquarium can kill a lot of fish. Jay Wilson lost a whole aquarium of fish when the floors were stripped in the room the aquarium was in. The air going into the aquarium needs to be pure air. Spraying for bugs can also create problems. Scents will not create a problem.

And if a house is tented one MUST remove the fish from the home prior to tenting. The gas used in tenting is very powerful and very penetrating and WILL kill any fish left in water in the home. This will happen even if the tank has the air pumps turned off and the tank is “sealed” with something like Saran wrap or aluminum foil.

Surface Area and Aeration

There are some old myths about aeration generally bandied about on goldfish forums and pond forums. They are along the lines of 12 square inches of surface area in the aquarium are needed per inch of fish length. This is a common myth.

First off it ignores the fact that a two-inch fish will NOT need twice the aeration of a one-inch fish. Rather a two-inch fish requires EIGHT TIMES the aeration of a one-inch fish. Secondly, it ignores the fact that a smooth as glass water surface will do about 5% of the aeration of a “choppy waves” surface of equal dimensions.

Another myth relates surface area to depth to find out if you have adequate aeration. The surface area to depth ratio is meaningless.

Units

If the water is FULLY saturated with air at 78 degrees F, it will be holding an oxygen level of 8.25 ppm (mg/L). Simply because the temperature is at 78 degrees the water will not necessarily be saturated. So the more aeration the better, regardless of the temperature. Note that warm water holds less oxygen than cold water.

Aeration in Greater Depth

Aeration is discussed at greater length in the following links:

9.2. Aeration and Turbulence

9.3. Air Stones

9.4. Skimmers and Spray Bars

9.5. Aeration and Temperature

Startpage Aquariumscience

Source: Aquariumscience.org – David Bogert