Oxygen is an indicator, or a symptom, of other issues. Too much aquatic plant life or intense plankton bloom, oxygen spikes high and low through a normal day. But, plants are the problem. Heavy organic load, decomposing plants, etc., oxygen drops. But, the decomposing organic matter is the problem. An oxygen meter or test kit allows you to measure oxygen at any given time. But, if something is wrong, and the oxygen meter says oxygen is too low...it's too late, unless you have specialized aeration equipment. Since you have a Koeneder's windmill and are keeping snow off the ice, you should be in good shape. Here's another important fact to remember. As water temperatures drop, its affinity for oxygen increases. Colder water holds more oxygen. So, unless you have an intense plankton bloom, or lots of underwater salad, I wouldn't worry too much about oxygen.
