Oxygen Requirements In Home Aquariums

Oxygen consumption in the home aquarium is a frequent topic among fish keepers, but do you really understand what is going on inside of your tank?

If you are a bit lost on this issue, you are in good company. It can be quite difficult to imagine something that you can't see, especially when the portions are so amazingly small. Can you really visualize just how tiny 7 parts per million is?

We hope that the following examples will be of some help.

Let’s say you have a 135 gallon tank that has been stocked with 5 lbs. of healthy fish. The water in the tank is kept at 79°F with the pH at 7. Like most people, you are a bit heavy with the fish food. Your fish are being fed 1% of their body weight per day (approximately 25 grams or almost 1/3 of a cup of dry food). Using long established formulas from aquaculture, this is how the math works out:

• Your 5 lbs. of fish will require 0.9 grams of oxygen per hour. This equals 21.6 grams of oxygen per day.

• These fish will produce 0.8 grams (several drops) of NH3-N per day.

• The amount of oxygen required by the bio-filter to convert this Ammonia to Nitrate and satisfy the requirements of the bacteria is 5.4 grams of oxygen per day.

• The total biological oxygen demand for your tank is 27 grams of oxygen per day.

Now let us consider a simple source for this oxygen.

Air stones are by far the least expensive and the most efficient way to aerate an aquarium. Aeration can take place in the tank, sump, filter or protein skimmer. It makes no difference where the oxygen is introduced into the system, just so long as it is present in sufficient quantities.

A small, high quality air stone (1.5"x .75") will flow slightly less than 0.10 cubic feet of air per minute. Three (3) of these stones will add 1.8 grams of oxygen to the water per hour, 43.2 grams of oxygen per day.

Oxygen saturation will occur somewhere between 6 and 8 parts per million (this number depends upon temperature, salinity and elevation). This means that the water molecules have absorbed as much oxygen as they can easily hold under the present conditions. Any oxygen taken in beyond this point will be released back into the atmosphere with the least amount of disturbance in the water.

75% oxygen saturation is common in the well aerated home aquarium, while 95% is quite difficult to achieve.

Since your aquarium has an oxygen demand of 27 grams, and the air stones are providing an oxygen potential of 43.2 grams, there is no danger of low oxygen stress. Your tank will utilize and store only a certain amount of the available oxygen. And while it is true that you may have a tremendous amount of surface area in your biofilter, it is highly unlikely that the bacteria will utilize all of it.

Remember, nitrifying bacteria are present in levels that are directly proportional to the bioload of the tank. It makes no difference to the bacteria how big your filter is. If they need only 10% of the surface area, that is what they will use. Approximately 80% of the tank’s oxygen demand goes to the fish, with the remaining 20% going to the bacteria for the nitrification of organic waste.

In conclusion, your average fish tank is not consuming nearly as much oxygen to process the biological waste as you may have imagined. While this is understood by professional aquarists, it has taken some time for the information to reach the aquarium hobbyist. Both wet/dry and fluidized bed biological filters utilize the same amount of oxygen to process fish waste.