I had a high number of egg deaths when I allowed the eggs to be unseparated in a container. I now use the tacklebox method (each egg SEPARATE) with calcined clay as a hatching medium. It is FANTASTIC and I did not have one egg death last year. I think the babies were knocking the other eggs around when they hatched, and it caused the developing babies inside to die. SEPARATE those eggs into individual compartments and use a different hatching medium--like for pond lilies, the calcined clay. It's incredibly easy to tell when it's dried out, as it gets very light in color, and when it's correctly saturated it's dark. Plus, when I previously used vermiculite, I had a baby take a mouthful of it and die inside the hatching container. That does not happen with the clay, period.
EDIT--Many places don't even know they have calcined clay, they don't know what all of the mediums are made of--You have to ask for pond lily planting medium.