Oxygen Basics
You and I, your cat, and your canary all breathe air containing 20.9% oxygen, whereas 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 increasing their consumption of oxygen. For every 10C there is an approximate doubling of the consumption of oxygen (and production of carbon dioxide). Your fish may be happy and healthy at 20C (68F) and suffering from low oxygen stress at 30C (86F) 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
* Warm water saturation is about 8 ppm
* Continuous healthy minimum is about 6 ppm
* Low level stress and poor feeding response 4-5 ppm
* Acute stress, no feeding, inactivity 2-4 ppm
* Death 1-2 ppm
The above is a guideline only, as duration, water quality, condition of fish, level of other gasses, 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), favor growth of disease causing pathogens and disrupt the function of your biofilter.
Solution
You can either design and maintain your pond so that oxygen never becomes a problem, i.e., 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 oxvgen 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.
Aeration
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, air diffuser 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 1.000 gllon (3.800 liter) pond.
1,000 gallons of water at 6 ppm contains about 24 grams of oxygen. 10 lbs. of fish need about 18 grams of oxygen per hour. 1,000 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, nonaerated 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 8:00PM, 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
Following is a list of typical Koi pond aerators with estimates of oxygen transfer performance and efficiency. (Assumes 3000, clean water, 70% of saturation, power cost $0.1 0/kwh.)
| Aerator | Flow rate | Watts used | Grams O2 per hour | Cost per 100 grams |
| Lazy Stream 12" drop | 10gpm | 150watts | 1.2 | $1.26 |
| Babbling Stream 12" drop | 10 gpm | 150 | 2.4 | .63 |
| Waterfall 12" drop straigh into pond | 10gpm | 150 | 1.4 | 1.05 |
| Waterfall 2-24" drop fallng on rocks | 10 gpm | 200 | 3.1 | .65 |
| Fountain | 5 gpm | 100 | 1.6 | .63 |
| water pump with venturi | 4 gpm | 100 | 4.0 | .25 |
| Air compressor linear type 1 | 1 cfm | 30 | 4 | $0.06 |
Summary
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 miting factor in water quality. Ammonia and nitrite take days to reach crisis levels. Oxygen can become critical in a few hours.
Source : http://akca.org/library/aerate.htm
