Don't use sand, here is an article.
http://www.fresh-eggs-daily.com/2013/07/the-real-scoop-on-using-sand-in-your.html
Is there enough ventilation in their coops. Think of how our homes are vented. I would be worried about the birds being to high and getting injured jumping down. You...
I wouldn't be adding any more chickens to your flock, its obvious you have a highly contagious disease going through your flock and will continue to kill off your birds. Any that does survive will be carriers. It is probably best to cull them all and find out what is going on, then start...