Fyfer - As long as you can keep similar amounts of snow removed from the ice you should not need to run the aerator during ice cover. Those fish that died during winter might have been real old fish that died from high body stress during long cold winters. Winters are very hard on the oldest fish or those near the end of their life that tend to get weak during hard winters.
Moving the diffuser head close to the surface and hang along the vertical wall during winter mixes less of the deepest bottom water. Mixing mostly the upper layer in winter still provides open surface water for light penetration, DO production and for less benefit of decomposition gas to escape. Mostly the surface water in winter provides or maintains the warm 39-40F (4C) water in the deep water. The shallower you can place the diffuser the more unmixed bottom water remains as a warm zone. Diffuser on the bottom tends to mix the entire water column producing super cool water to temps below 4C. Fish will have less cold stress if 39F water is available.
That makes sense regarding mixing the water temperatures. We did however install the aerator as recommended by a pond manager not for the fish but to prevent spring weed growth by allowing gasses to escape. Do you have any comments on how effective the aerator will be at preventing spring weed growth, which is a major problem in our pond. We have curly leaf pondweed which we have discussed before. You had recommended various herbicides such as diquat but they are simply impossible to acquire or apply here, so this was our only choice. Thanks again.
