Sorry to hear that Jim. I have heard similar stories from people here. Also same thing that happened to you, mixed Asia fish in with domestic stock... problems with pups dying ever since. People don't want to believe that there are problems with some Asia fish, but it's good people like you come forward and go public about issues. So that the evidence sees the light of day.I myself am going through a similar problem
I have lost 14 pups this year all to very stringy white discharge. They last from 3 days to a week or so before dying.
This all started when I introduced an Asian import ray to my European bred stock.
I too have a pup out for autopsy and I am awaiting results.
The general consensus so far is that it's a virus or pathogen. The domestic adults are usually strong enough to adapt to it over time... I'm sure your adults showed signs of stress when first being mixed together. As time goes on they can build up immunity to it. However like you said, the pups when born are too weak to handle it and die within a short time.
From what I have heard from a few large scale breeders, nothing has been found to be effective against this. Only over time, it will pass. Wish you the best Jim. Hopefully you can find some new information.