More fish die from a lack of oxygen than any other cause
requirement is generally believed to be a concentration of around 6 parts per
million (PPM) of oxygen in the water.
In 1959, A. Krogh, in his "The
Comparative Physiology of Respiratory Mechanisms", showed that a 200 gram Koi
requires 100 millilitres of oxygen per kilogram per hour. His tests on a 70,000
gram man showed we require 200 millilitres of oxygen per kilogram per hour at
rest, which jumps to 4,000 ml/kg/hr at maximum work levels.
So Koi need
a lot of oxygen, almost as much as we do when resting.
To be educated
fish keepers we must have a clear understanding of oxygen. We need to know what
takes oxygen out of the water and how to put it back.
You and I, your cat, and your canary all breathe air
containing 20.9% oxygen, where as your fish breathe water containing 0.0008%
oxygen! They have evolved to be comfortable with that amount, but at 1/2 that
amount, 0.0004%, they are hurting! In garden ponds we typically see oxygen
problems only during the summer because when the water is warm all those things
which consume oxygen speed up their consumption of oxygen. For every 10°C
there is an approximate doubling of the consumption of oxygen (and production
of carbon dioxide). Your fish may be happy and healthy at 20°C (68°F)
and suffering from low oxygen stress at 30°C (86°F) in the same pond.
Oxygen levels can only be determined by measurement with a test kit or an
oxygen meter. Submerged plants and algae take oxygen out of the water at night.
Their effect can be significant when you have "green water" (phytoplankton
algae and zooplankton).
- Warm water saturation is about 8 ppm
- Continuous healthy Minimum is about 6
- Low level stress and poor feeding 4 - 5
- Acute stress, no feeding, inactivity 2 - 4
- Death 1 - 2 ppm
| The above is a guideline only, as duration,
condition of fish, level of other gases, etc.
all have a
Low Oxygen Stress
It is important to know you have an oxygen problem
long before your fish start dying. You need to either monitor it by
periodically measuring it when you expect it to be low or, select and use an
aerator large enough to handle the summer time conditions. If you are stressing
your fish, you are making them much more vulnerable to disease, parasites and
infection. Their activity level will be reduced as well as their growth rate.
Low oxygen levels will lower the oxidation/reduction potential (ORP), favour
growth of disease causing pathogens and disrupt the function of your biofilter.
You can either design and maintain your pond so that oxygen
never becomes a problem, ie: clear water, few fish, little food and clean
bottom. Perhaps, use an aerator, at least during the warm periods. Since every
pond is unique and conditions, especially water quality and fish quantity vary
greatly, I can only give you general guidelines (see oxygen budget). If you are
not sure - measure the oxygen level. Buy or borrow a dissolved oxygen test kit
or an oxygen meter. Use it along with your preferred aerator until you are sure
that the oxygen level will be maintained.
Most fish keepers know they need some type of aeration. But,
unless you've studied this science (and who has?) you are probably assuming
that your stream, venturi, waterfall, or fountain is taking care of your
aeration needs. They may be, but are your sure? Almost anything that assists
the transfer of oxygen into water could be called an aerator. But is the
aeration effect significant and is the energy expended cost effective? The
oxygen content in your pond depends upon the rate of consumption vs the rate of
The need for more oxygen comes from fish, plant
respiration, the chemical and bacterial decomposition of waste matter. If you
really want to read the numbers, read the following:
budget for a 1000 gallon (3800 litre) pond. 1000 gallons of water at 6 ppm
contains about 24 grams of oxygen. 10 lbs. of fish need about 18 grams of
oxygen per hour. 1000 gallons of water with a chlorophyll A of 20 mg/l (light
green water), a B.O.D. of 10 mg/l (uneaten food, bacteria, etc.) will consume
about 2 grams per hour. Feeding 3% of the fish's body weight per day will add
60 grams of food which contains 24 grams of protein, which converts to about 2
grams of ammonia, which will consume about 14 grams of oxygen in the biofilter.
Pollen, bird droppings, leaves, etc. may add 2 more grams. 36 divided by 24
equals 1.5 grams per hour.
This example shows the fish consuming the
most oxygen and the biofilter the 2nd largest amount, but be aware that very
green water can cause a fish kill in a stagnant, non aerated pond even if it
contains only very few fish! If you had an aerator maintaining the oxygen level
at 6 ppm and you turned it off at 8:00 PM, you would lose about 6% (1.5 grams)
of the pond's oxygen per hour. By 8:00 AM, the next morning, the oxygen level
could be as low as 1 ppm. If your aerator is your fountain, do not turn it off
The following is a list of typical Koi Pond
Aerators using water pumps with estimates of oxygen transfer performance and
efficiency. Tests were done using "State of the Art" electronic meters for
(Assumes 1000 Gallons, Clean Water, 70% of
Saturation of Oxygen)
|| FLOW RATE
of Oxygen Made
|Lazy Stream - 12" drop
||10 gpm or 600 GPH
||1.2 grams of
|Babbling Stream - 12" drop
|Waterfall - 12" drop straight into pond
|Waterfall - 24" drop falling on rocks
|Water Pump with Venturi attached
| *Air Compressors are Equal in Grams/Hour/Oxygen as the Venturi
But in shallow water of less than 10' you'd need multiple units to cover a
large area. Most ponds DO NOT use air compressors which will cost about $350 on
average and if you already have a pump the Venturi is what we suggest as an add
Most importantly Airstones Will NOT create Current &
Circulation or Exercise your Fish, move bottom waste to your filter or drains
and in Depths of Less than 10' they will not Mix the entire pond volume unless
multiple units are used. The worse problem is the fact that they will Greatly
Obstruct your view of the water and Fish which our units do not.
You will use less electricity with a compressor but the
Advantages of Venturi Aeration far outweigh the cost saving of
Make sure that your fish are not being stressed. If your
fish are valuable to you, it is cheap insurance to have two aerators or
separate breakers. If one fails, the other will prevent severe fish stress or
If you are building a pond, design it so you never have
to worry about oxygen. Wire it so that your water pump and aerator are served
by separate breakers, select energy efficient long life components. Consider
that someday you will have a lot of fish, lots of algae, warm water and a
broken pump. Oxygen is the first limiting factor in water quality. Ammonia and
Nitrite take days to reach crisis levels. Oxygen can become critical in a
We normally suggest running our product at 1000 GPH to
3000 GPH which is needed to clean the pond bottom and will create a slow
current flow & Mix the layers of water which also increases Oxygen Levels
in addition to the aeration. So if the 4 GPM which translates to 240 GPH, in
the chart <A> above would increase to 8 GPM with a 1000 GPH pump this
would result in an increase of roughly 8 grams/hour of Oxygen added <C>
and the cost would still be Lower than the 24" Waterfall delivering only 3.1
gm/hr of Oxygen <C>.
When using a Venturi the water is forced through
a small orifice opening so a 500 GPH pump will only use about half for aeration
(250gph/4gpm) but doing so cuts the power to run the pump. Then to double the
GPM above would require about 1000 GPH depending on the PSI or pump pressure