First let me say my condolences are with you.
> Or maybe a combination of 200 litres of new unoxygenated water, close to
> lights-out (night respiration load), plus an airpump failure.
I've given all possibilities some thought and I think you hit the nail right
on the head here. Also water of 29-30 degrees can't hold much oxygen. It was
the combination of these 3 factors that did it.
> Trouble is that no option seems to be totally consistent with the outcome.
This one is. The larger fish require more oxygen and would die first. It was
no pathogen or bacteria, the symptoms don't match. It was also no chemical
contamination, that would also have killed most of your corys.
Best regards,
Hugo.