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More fish die from a lack of oxygen than any other cause

Koi's oxygen 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.

Oxygen Basics

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).


Oxygen Levels

  1. Warm water saturation is about 8 ppm 
  2. Continuous healthy Minimum is about 6 ppm 
  3. Low level stress and poor feeding 4 - 5 ppm 
  4. Acute stress, no feeding, inactivity 2 - 4 ppm 
  5. Death  1 - 2 ppm 
The above is a guideline only, as duration, water quality,
condition of fish, level of other gases, etc.
all have a significant effect

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 replacement.

Oxygen Budget

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:

Theoretical oxygen 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 at night.

Aerator Performance

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 Oxygen measurements.

(Assumes 1000 Gallons, Clean Water, 70% of Saturation of Oxygen)
of Oxygen Made
Lazy Stream - 12" drop 10 gpm or 600 GPH 150 watts 1.2 grams of
Oxygen Produced
Babbling Stream - 12" drop 10 gpm 150 2.4
Waterfall - 12" drop straight into pond 10 gpm 150 1.4
Waterfall - 24" drop falling on rocks 10 gpm 200 3.1
Fountain 5 gpm 100 1.6
Water Pump with Venturi attached 4 gpm 100 4.0
*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 on.

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 compressors.


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 mortality.

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 few hours.

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 available.

Acknowledgements to www.bestkoifish.com/