Rachel,
So sorry to hear about your loss!
The biologist inside me really wants to understand what is going on inside when this happens. I wonder if we just don't understand the nutritional requirements for these fish enough. When a fish looks and acts perfectly fine one day and is dead the next - still looking fine...I wonder if you could examine their liver or other internal organs if we'd find that we've kept their bellies full but they've been missing something.
Could you please tell us if Mollies were the only food Cow has been getting or if you have also fed other foods? I keep hoping that if we gather enough hobbyist information, something will click.
Again, my condolences,
Cheri