alot of people put eggcrate under there rock and then fill in and around with sand afterwards...heres a trick to no sand storm:
take a length of pvc or abs ppe(the wider diameter the better) cut it at a length where it can go all the way to the bottom of the tank , keeping the pipe straight so the bottom is flush fill the pipe with sand, slowly lift the pipe and your sand will come out slowly, you can also have sand where you want it in any amount without a dust storm, but rinse the sand first
if your adiment on putting the sand in before rock then put all the sand in and cover with a garbage bag and use dinner plates to hold the bag down, pour the water over the bag ,when the tank is full lift plates and bag slowly and then add rock