Changing substrate

I was looking for a few opinions on changing substrate. My parents have a 125g that they originally setup with crushed coral. They got me into the hobby and I went with various sands. They wanted to do the same. Their tank had been up for close to 2 years and werent sure how to go about it. Took all the rock out and put it in bins, and drained a decent amount of water. Then did the switch. They had 3 clowns, a yellow tang, and a blue hippo tang. They did this early Saturday and after both tangs appeared fine. But lost them both suddenly. Wondering if it was just stress, stirred something up in the water, parameter off? Or what might have been the cause.

 

Nope they were worried since they died so quick. The change was early saturday. After they seemed fine, I think they said sunday morning they were fine. Then all of a sudden the yellow tang was dead and the blue hippo wasnt doing good and died shortly after. Other than that the clowns seem to be ok and corals seem to be ok.