If you don't add the sand very soon, you're going to have a problem..also hope you have a cover on that tank!
Trust me on this..the Yellowtail Coris will move ANYTHING THAT IS NOT GLUED DOWN. There are days I want to strangle mine...he's killed 2 clams by tearing them off their rocks...just so he can see if something tasty is underneath.
Be sure to always research the fish you buy before you purchase them!