Technical Talks


  • The First Limiting Factor of Water Quality

    Air breathing animals are used to air containing about 21 percent oxygen, whereas aquaculture is conducted in water containing less than .0001 percent oxygen (10 mg/L). With so little oxygen available in the best of conditions, it is apparent that knowledge of dissolved oxygen (D.O.), oxygen measuring and aeration equipment is very important to the aquaculturist.

    If the oxygen level is too high, oxygen supersaturation can cause gas embolism, depress metabolism and inhibit respiratory enzymes, all of which can kill fish. If it is too low, the fish may not eat and may expend additional energy seeking oxygen. If lower yet, they can experience severe stress and, of course, death.

    The diurnal oxygen cycle in outdoor ponds is show below. The delicate balance of dissolved oxygen vs the rate of oxygen consumption can shift rapidly with changing algal, temperature and wind conditions. In outdoor ponds, lower D.O. levels will occur in the summer because the rate of oxygen consumption increases as the temperature increases. Oxygen is THE MOST IMPORTANT thing to monitor. Monitoring and record-keeping will provide predictive knowledge.

    Measure oxygen after altering water flowrates, feed rates, etc., and when developing new growing systems. No one can just look at the water and know the oxygen level. Oxygen can only be measured with a test kit or, more conveniently, with an oxygen meter.

  • Incorrect pipe sizing all too often causes unsatisfactory performance. Friction is the culprit. As the volume of air passing through the piping increases, the pressure required to deliver the air also increases due to friction (think of it as wind resistance). Since most aquaculture type aeration systems utilize low-pressure blowers, it is critical that nonrestrictive piping be used.

    Pressure loss in air systems can be measured in inches of water ("H2O). The resistance to air flow caused by friction will decrease both the pressure to the air outlet (typically an air diffuser) and the volume of air delivered.

    When designing the air system, it is important to add together all of the following: the maximum water depth to which the air is driven, the resistance in the air piping system and the resistance caused by the air diffuser. Our simplified chart can be used as a guide in determining the pressure loss caused by the piping. The blower pressure and air diffuser resistances are published in this catalog. If you are confused, don't worry. Call an AES technician at 407-886-3939 for help.

    Example: 4 cfm need to be delivered a distance of 200 feet from a rotary lobe blower. The average line pressure is 3 psi. There are no odd twists or elbows that need to be considered. The minimum diameter of plastic pipe will be 3/4", causing 7.4" H2O resistance or pressure loss. The smaller 1/2" pipe would cause 24.6" of loss, which would probably be unacceptable. A 1" pipe, costing little more than the 3/4", might be an even better choice if there is the possibility of using more air in the future.


  • Anyone who has worked with fish for a while knows that if you don't plan ahead, you won't be in business for long. When it comes to life support, a backup blower could save your business.

    The most crucial of life support requirements is maintaining an adequate oxygen level because it can be used up so quickly.

    We always recommend having a second blower attached to the main air supply line, wired to come on if the primary blower fails for any reason. Both blowers must have check valves on them. A pressure switch is located between the primary blower and its check valve. When this pressure switch senses a loss of air pressure, it closes, causing an electric relay to start the backup blower (see diagram). The check valves keep air from being lost through the nonrunning unit.

    Check valves must be able to tolerate high temperatures. Be sure the stand-by unit is on a different electrical circuit breaker.

    The cost of fish food can represent 30 to 90 percent of the total cost of raising fish! In some situations (like indoor culture), feed must provide 100 percent of the complex nutritional requirements fish need. A less-than-complete diet will show up as poor feeding response, slow growth and/or disease problems. Poor water quality can also be caused by bad feed.

    All feed should be used within the normal 6-month shelf life. Do not overfeed. In general, give fish a little less than they will eat (stop feeding them before they stop feeding).

    Questions often arise about the size of fish food. Here are some suggestions for selecting food that is the right size for your fish.

    • Some smaller hatchlings require food as small as 35 microns! Other hatchlings may be able to accept brine shrimp, large zooplankton or starter crumble from the onset. When in doubt, use the smaller size food.
    • When changing food sizes, it is best to do so gradually by mixing the two sizes together, slowly converting to the larger size in a week or so.
    • To learn more about feed sizes and feeding practices, consult the book Fish Hatchery Management (see Index).

    Getting Started Guide:

    • For fish under an inch in size, use brine shrimp, rotifers, powdered foods, flake foods or 00 starter crumble.
    • To 2 inches: 1/32" granulated food, zooplankton or flake foods.
    • 2 to 3 inches: 1/16" granules, zooplankton or flake foods.

    As fish grow to 3 inches or more in length, food sizes become more uniform by species:

    • 3 to 5 inches: 3/32" or 1/8" pellets.
    • 5 to 10 inches: 3/16" to 1/4" pellets.
    • 10 inches: 1/4" to 3/8" pellets.


  • Efficiency versus Type ...

    Select the right type of aerator for the application.

    An aerator's standard aeration efficiency (SAE) is an important consideration when comparing one aerator with another.

    The SAE can be calculated by measuring the aerator’s oxygen transfer and the amount of energy used per horsepower, per hour, under standard conditions. An SAE of 2.1, for example, means that 2.1 pounds of oxygen per horsepower, per hour, are transferred to the water under standard conditions. The higher the SAE, the higher the oxygen transfer, the higher the efficiency.

    However, SAE numbers are a fair comparison only when comparing aeration equipment of the same exact type. You cannot use the SAE as your only tool in the selection of an aeration system. Before looking at the SAE numbers, choose the right type aerator or oxygenator for the job.

    Here are some examples (analyze the differences):

    • A surface aerator like our Kasco 3/4-hp (with an SAE rating of 2.9) may be a good choice where the volume of water is small and the stocking density is high, such as in a culture tank or small pond. It may be a poor choice, however, if cold water temperatures are required when air temperatures are warm (summer trout culture), because both the motor and the airborne water droplets will add unwanted heat.
    • A surface aerator would be a poor choice for a large or deep pond. Without moving water away from the aerator, it will continue to pump the same water over and over again, adding no oxygen where it is needed.
    • A surface aerator may be excellent in emergencies because it quickly raises the oxygen level in a small area. If trained, the fish will move to that location. Again, it will not be a good choice for full-time aeration because it will not disperse oxygen throughout the pond.
    • A diffused air system (with an SAE rating of 2.7) may be the best choice for multiple tanks and ponds because the energy source (blower) can be centralized and just the right amount of energy (compressed air) can be easily directed where it is needed. What appears to be lower SAE efficiency (2.7) is more than offset by comparative application efficiency.
    • Water-moving aeration devices like the AIRE-02® Series II aerator and paddle wheel type aerators are excellent choices for medium and large ponds where movement of oxygenated water away from the aerator is most important.
    • If destratification alone will solve a bottom oxygen problem, only a few air diffusers may be needed to accomplish this. For instance, a 10-acre lake, 15' deep, may need only 3/4 hp (see Lake Aeration). Only 1 cfm of air may be needed to aerate a 1/20-acre pool when raising tropical fish (see FAT CAT™ aerators), compared to 6 cfm per pool using airlifts.
    • A destratification system, such as our Great Lakes® aeration system, should not be used as an emergency aerator because it very quickly mixes the water. Its rate of oxygen transfer is excellent; however, it cannot raise the oxygen level of such a large volume quickly enough to avoid a fishkill. It can actually aggravate the problem.
    • Even noise is a consideration above and below the water surface. Pentair AES diffused air systems are preferred over agitators in bait stores, improving conditions for both the fish and the employees. As more of our research is conducted under water, we are increasingly aware of underwater noise produced by devices such as paddle wheels, agitators, airlifts and drilled pipe spargers (our Sweetwater® air diffusers have a barely audible hiss). Some fish breeders have reported that diffused air is the only aeration method they can use that will not disrupt breeding. The situations and considerations are virtually endless, so take some time to research the best aeration method for your particular application. For the price of a phone call, you can discuss your application with a Pentair technician and get an expert opinion (877-347-4788).

    Standard aeration efficiency tests conducted at Auburn University showed that the Pentair AES Sweetwater® diffuser and 1-hp blower combination yielded a 2.71 SAE. Prior to rating our diffusers, Auburn gave air diffusers in general an SAE rating of only 1.6. That 70 percent increase illustrates how much efficiency can change when the right combination is used.

  • You know how you can tell what people know by the questions they ask? Well, we know that most people don’t know about watts. They ask, "How many amps does this motor use?" instead of, "How many watts does this motor use?" Watts are what you pay for, not amps (amps are used to size breakers, etc.).

    The direct current formula we all learned (volts x amps = watts) is correct for incandescent light bulbs and electric heaters, but it is not correct for motors. When dealing with power loads that involve inductance magnetic devices such as motor windings, solenoids, transformers, lamp ballasts, etc., the formula for single-phase loads is volts x amps x power factor = watts.

    In many cases, especially with linear air compressors and mag drive pumps, the actual watts used are significantly less than what is calculated by multiplying volts x amps. The only way to determine the watt consumption of a motor is to test it using a wattmeter (such as our KW4). In the Pentair AES catalog, we have published the actual watts for most of our motor-driven devices as tested in our RDTE shop with our wattmeter.

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