I'm donno what was your setup and your water cycle....but
I think it was high ammonia which very toxic that had caused the killing.

Without greenwater or beneficial bacteria (or other biofilter) the ammonia will increase and remain untreated. Even small amount of ammonia is dangerous to your fish, much more for the babies.

Should water cahnge be done earlier, this maybe avoided.

Hope this will not happen again....good luck.