Hello-and welcome to PondBoss.
Not an expert here, but from what I've read, and remember from organic chemistry, you're almost certainly having a DO crash, and your most stressed fish, the large postspawn YP, are most vulnerable.
Your algae are dying, therefore not producing 02 and in fact consuming it as they degrade. Not sure about your Cutrine, but some copper products recommend only treating part of the pond at a go. Although completely logical and understandable ["Nontoxic to Fish!!"], you probably magnified the crash by running the aerator full bore to fully disseminate the cutrine.
So, in terms of recommendations--probably best to run the aerator all the time for now.
