All About Dissolved Oxygen

The earth's basic air envelope is made up of about 78% Nitrogen, 21% Oxygen, and 0.03% Carbon Dioxide. There are also traces of several other elemental and molecular gasses but they will be ignored since they have no known effects within the pond environment. Concentrations of these gases within water is a whole different story. The concentrations are much smaller and are measured in milligrams per liter (mg/l) or somewhat equivalently, in parts per million (ppm). A typical pond at a temperature of 70° F. will have concentrations of about 13 mg/l Nitrogen, 9 mg/l Oxygen, and 35 mg/l Carbon Dioxide. As the air components dissolve into the water, a point is reached where no more can be added. This point is called saturation. The saturation points are different for each of the gases and are dependent upon several different factors but temperature is the most important. As the temperature increases, the water simply cannot hold as much of each type of gas. For oxygen, the approximate saturation level at 50° F. is 11.5 mg/l, at 70° F., 9 mg/l, and at 90° F., 7.5 mg/l. (See Figure 1) Impurities added to the water (i.e. salt) further decrease these saturation levels. Four pounds of salt per hundred gallons of water (5 ppt) will decrease the oxygen saturation levels about 1 mg/l.


Oxygen Saturation vs Temperature
Oxygen Saturation Graph

Figure 1



Fish are remarkably well adapted for extracting oxygen from the very low concentrations found in water. The rate of oxygen consumption by Koi is closely related to the water temperature. Koi are "cold blooded", that is, their body temperature is essentially that of their environment. Their metabolic activities are basically enzyme-catalyzed chemical reactions that are temperature dependent. The metabolism and activity increase with temperature which increases their oxygen demand. There is both an optimum and maximum temperature at which the Koi live and function. At optimum temperature, oxygen consumption is high because of rapid growth and significant activity. Above this optimum temperature, the fish start to experience stress. This stress triggers their warning and defense systems which require a very high oxygen consumption. Unfortunately, as we saw above, the amount of oxygen available in the water also decreases with temperature. The combination of these two events normally limit the maximum temperature at which the Koi can survive.


The minimum limiting oxygen concentrations for a fish is dependent upon its genetic makeup, water temperature, level of activity, long term acclimation, and stress tolerance. Water with an oxygen concentration of less than 3 mg/l will generally not support fish. When concentrations fall to about 3-4 mg/L, fish start gasping for air at the surface or huddle around the water fall (higher concentration points). Bio-converter bacteria may start to die off dumping toxins into the water compounding the lack of oxygen to the fish. Levels between 3 and 5 mg/l can normally be tolerated for short periods. Young Koi are less tolerant of low oxygen than the older, larger ones. Above 5 mg/l, almost all aquatic organisms can survive indefinitely, provided other environmental parameters are within allowable limits. Whereas the fish are reasonably comfortable and healthy at 5-6 mg/L concentrations, many people consider the efficiency of the bio-converter to be at maximum only when the water entering the bio-converter media is near oxygen saturation. Ideally, our ponds should be at or near oxygen saturation at all times.


Pill, powder, and droplet (or combination) test kits are available. Most involve three steps and a final colormetric chart. Recommended test kit range 0 - 15 mg/L. Note: Some test kits can show false readings if various chemical treatments are in the water. Electronic Dissolved Oxygen meters are also available. These are accurate and convenient, but quite expensive. A Dissolved Oxygen test kit is considered nice to have but not required for the average pond.


Whenever air is in contact with the water, whether through natural or artificial means, a transfer of oxygen from the air to the water takes place until the water becomes saturated. Plants under light, convert carbon dioxide to oxygen in the water. Fish, plants at night, and aerobic bacterial action consume the oxygen.
Koi Fish Gasping for Air


It is not difficult to get all the air into the water that the fish need. Oxygen is continually transferred into the water at the surface of the pond and normally only a small water fall will bring the pond water to or near to saturation. Heavily populated ponds may need supplemental air and ponds with a large amount of algae may need supplemental air at night when the plants are not making oxygen but consuming it. It is very important that sufficient circulation is provided within the pond so that all areas have proper oxygenation.


Almost all of the oxygen dissolved into the water from an air bubble occurs when the bubble is being formed. Only a negligible amount occurs during the bubbles transit to the surface of the water. This is why an aeration process that makes many small bubbles is better than one that makes fewer larger ones. The breaking up of larger bubbles into smaller ones also repeats this formation and transfer process.


A "sheet" type waterfall can provide more dissolved oxygen in a pond than the "cascade" type waterfall whose velocity is low when the water finally enters the pond. Although the cascade type waterfall provides better aeration of the water that is entering the pond, the sheet type provides better aeration of the water that is already in the pond. The sheet of water tends to shear the larger bubbles of air formed at surface entry into smaller ones below the surface. This action can occur at depths of up to three feet or more and result in oxygen transfer to a much larger amount of water than just that which is entering the pond. For most situations, the amount of water flow is determined by filtration requirements and either type will be more than sufficient to maintain the pond oxygen levels at or near saturation.


A common method of providing additional oxygen to the water is through the use of an eductor type air jet (sometimes called a venturi). An added advantage of this device is that it can simultaneously provide improved circulation of the pond water.


Air stones or similar bubble forming devices driven by an air pump can also be used to provide supplemental air. A single air stone can supply sufficient air for up to a 1000 gallon pond although pond water circulation problems may still exist. It is recommended that a backup air pump with tubing and air stones (size and quantity depending on pond size) be kept on hand in case of main water pump malfunctions. This could also be used to supply air to an isolation tank if needed. In an emergency, just splashing the water by hand or with a bucket can probably add enough oxygen to sustain the fish until the problem is corrected.


When a power loss or other malfunction causes water flow to stop and hence most aeration to also cease, several problems develop. The oxygen concentration drops and ammonia starts building up. The size and population density of the pond will determine how long before this becomes a problem but the bacteria in the bio-converter will start dying off at about the 4 hour point. After about 4 hours, it is important that before circulation through the bio-converter is restored, that it be drained to remove any toxins released by the dying bacteria. The ammonia levels and nitrite levels should then be monitored closely for a couple of days.


Oxygenation During Transport

When plastic bagging fish for transport, use only enough water to just cover the dorsal fin. Squeeze out the current air, add 5-10 times the amount of oxygen as water. This is normally sufficient oxygen for up to 6 hours (if oxygen is not available, just plain air in the bag is sufficient for an hour or two).


Ammonia build up and temperature control then become the major problems. Based on controlled experiments, (the experiments were conducted using bass, trout, and carp but it is assumed that the results also apply to Koi) it was found that floating the transport bags in the pond for 30 minutes prior to release decreased the mortality rate due to temperature shock, particularly for small fish. This test was conducted with the fish bagged for one hour. For fish that had been bagged for four hours, it was found that the mortality rate increased for all sizes of the fish if the bag was floated for 30 minutes. My recommendation is that if the fish have been bagged for two or more hours, it is better to release them immediately than to subject the fish to the "bad" water in the bag for an additional half-hour. Thirty minutes of floating will prevent a sudden shock if the temperature difference is large, but it will not acclimatize the fish to the new temperature. Actual temperature acclimation of a fish takes several days, similar to us dealing with jet lag. It is not only the temperature the fish needs to be accustomed to but also the pH, hardness, alkalinity, "the taste", etc. of it's new surroundings.

If a transport tank is being used for moving fish, an air stone or aeration column can be used. A venturi (air ejector) is not recommended since the strong currents induced make the fish have to "work" harder which increases both the oxygen consumption and, of more importance, the ammonia waste products in the small tank. An air stone can be fed directly from bottled oxygen or from a small air pump. An aeration column can be fed from a small submersible water pump ideally located at the opposite corner or end from the aeration column.


CAUTION: Make sure that the transport tank's air supply cannot be contaminated with the vehicle's exhaust. Carbon Monoxide is very soluble in water and can be even more deadly to the fish than to you.

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