Oxygen Requirements In Home Aquariums
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.
Letís 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 79įF 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
- 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
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
tankís 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.