This is Ponds 101 and y'all probably already know it. I'm suspecting the water meal. Plants emit O2 in the daytime and suck it at night. Get 3 or 4 days of low sunlight via clouds and the plant life, including microscopic algae, keep sucking it. The bigger fish seem to go first from DO crashes.
Plus if it's 50% covered, that is 50% of the pond water that is NOT getting sunlight to grow phytoplankton, which puts more O2 into the pond than anything else. My bet is the Watermeal, not a 5�F temp drop.
What everybody is missing is this young pond is loaded with Watermeal. WHY????? A pond that age shouldn't have that problem. There are more underlying problems that aren't being addressed.