Hi Richard,
I think that there is no need to offer food until the yolk sack is exhausted, this would only influence the water quality negative and the pups will need good water conditions now. Turning out the light is for sure also a good idea!
But what I think is the most important: Remove the sand immediately!!!
The sand will damage the yolk sack, which will cause an infection and a quick death! I recognised this issue with premature born Scobina pups.
How big are those pups and how big is the yolk sack? Do you maybe have a foto of the already died pups underside?
Regards
Stefan