“Three opens” refers to turning on the aerator at noon on sunny days, in the early morning the day after a cloudy day, and in the middle of the night during prolonged rainy periods. “Two does not opens” means not turning on the aerator in the evening and not turning it on at noon on rainy days.

Three-way switch

1. Turn on the device at noon on a sunny day.

During the hot summer and autumn months, the aeration device’s water-churning action can break up the uneven vertical distribution of dissolved oxygen in the pond, promoting convection between the upper and lower layers of water. This stirs the large amount of supersaturated oxygen produced by photosynthesis in the upper-layer phytoplankton down to the bottom layer, thereby accelerating the decomposition of organic matter at the bottom and helping to reduce or prevent fish from floating to the surface early the next morning. It is generally best to operate the aeration device for one to three hours around noon.

2. Power on the next morning under cloudy skies.

On cloudy days, the upper layer of the pond receives poor light intensity, resulting in reduced oxygen production by phytoplankton and low dissolved oxygen levels in the water. Meanwhile, oxygen consumption factors tend to increase relatively. By early the next morning, the supply of dissolved oxygen becomes insufficient. At this point, turning on the aerator can fully harness its aeration and oxygen-increasing functions, accelerating the release of oxygen-consuming gases from the water and directly boosting oxygen levels, thus preventing the pond water from experiencing a sharp drop in dissolved oxygen. The aeration period typically lasts from around 3 to 5 a.m., usually continuing until sunrise.

3 consecutive days of rainy weather—turning on the device in the middle of the night.

Prolonged rainy weather not only weakens the photosynthesis of phytoplankton but also brings about low wind speeds, low atmospheric pressure, and high oxygen consumption, often causing fish in the pond to surface at midnight. Turning on the aeration system at midnight can increase dissolved oxygen levels and alleviate the problem of fish surfacing. Once started, the aeration should continue uninterrupted until sunrise. Additionally, if factors such as excessive feeding or a high fish population lead to severe surfacing of fish, it’s advisable to turn on the aeration system well before the fish start surfacing, thereby preventing the occurrence of “pond overflow” and fish mortality.

Neither opens

1. Don't turn on the device in the evening.

In the evening, the dissolved oxygen level in the pond is usually not yet critical, so there’s generally no need to turn on the aeration equipment. If you do turn it on in the evening, not only will its effect be minimal, but it could also cause the upper and lower water layers of the pond to mix prematurely, accelerating vertical mixing and increasing the depth of oxygen-depleted water. This can lead to an oxygen deficit, causing the dissolved oxygen level in the water to reach its lowest point by dawn—thus making fish prone to surfacing for air. Therefore, it’s best not to turn on the aeration equipment in the evening.

Don't turn on the device at noon on rainy days.

Because of insufficient light on cloudy and rainy days, the thermal stratification in the pond water is not obvious. Moreover, phytoplankton photosynthesis is relatively weak, so the dissolved oxygen level in the surface layer does not become supersaturated. In this situation, turning on the aeration equipment not only fails to increase the dissolved oxygen in the lower layers but also reduces the oxygen-producing capacity of the phytoplankton in the upper layer, thereby expanding the oxygen-consuming water layer in the pond and accelerating the rate of oxygen depletion in the lower layers. This can easily lead to fish surfacing for air, making it pointless to turn on the aeration equipment at noon to stir the water.

This is the traditional “three on, two off” approach, and the basis for making these decisions is the daily pattern of dissolved oxygen levels in still ponds, which dictates when to turn aeration equipment on and off. But how exactly do we determine whether the dissolved oxygen level in the water is high or low? In the past, aquaculture farmers relied solely on their own farming experience—there was no scientific basis for deciding when to start the aerators, how many to run, or for how long. As a result, it was impossible to confirm whether the dissolved oxygen levels in the water were truly sufficient. Moreover, in an effort to ensure “sufficient” dissolved oxygen, farmers often kept the aeration equipment running continuously, inadvertently driving up electricity costs.

However, the author believes that as stocking densities continue to rise, aerators will play an increasingly important role. In addition to the “three on, two off” rule, it’s also crucial to pay attention to the operating duration of aerators on hot days: during such days, aerators should run longer to enhance water exchange between upper and lower layers, thereby helping to cool the water and stimulate feeding. On cooler days, aerators should run for shorter periods. At midnight, aerators should run longer; at noon, they should run for shorter durations. When winds are light, aerators should run longer; when winds are strong, they should run for shorter durations. If the pond receives heavy fertilizer application, aerators should run longer; if the fertilizer application is light, aerators should run for shorter durations.