GSS,
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The subject of winter aeration, which is especially important for us northerners, has been a topic of past discussion. Here is one link to some good information.
Winter aeration Your initial post does not mention whether your air diffuser is located in the bottom of your pond. If you want to aerate your pond in the winter, given your geographic location, it is recommended that you place your diffuser in shallow water. Like H2Ofwlkillr mentioned, running a bottom diffuser in winter will result in mixing the warmer water from the bottom of the pond with the cooler upper layers. This situation becomes very stressful for fish (see Bill C's post in the link), especially warm water species like LMB which may be the case with your LMB loss. There is a post out there that explains this concept with more detail than I can provide. Perhaps some will post the link, if not you'll have to go fishing for it.
Here is another link involving winter aeration
Winter aeration Russ
