if you had a deep sand bed of any more than 2", then moving it and disturbing the bottom layers could have released both detrius and possibly sulfur compounds (both of these could have resulted in fish death) that are present when denitrification is present. disturbing sand beds with out washing it out and letting it dry is problamatic IME. i did not read if you had a DSB or not or if you washed out the sand but it is possible that it could have caused a mini cycle like Angie suggested.