sand

That's what I did, and I had great success with the live bagged sand. Some people may bash it but my experiances are of positive ones. Just make sure to have your live rock out of the tank when you add the sand, I didn't do this and I'm still shooting sand out of the holes in the live rock with a turkey baster. Another option of adding sand is put the entire bag in the tank fill it with water and gently slide the sand in the location you desire trying to evenly spread it through out the tank.

Also the sand really helped my copepod population flourish, which is what you want to see when starting a new salt tank! Good luck, keep us posted Pics are ftw!