It depends if you have live rock or imitation live rock. Live rock is not MarcoRock, or Reef Saver, or that tub of rocks at your LFS. And if you want to go without sand, first, best of luck, but you better have well established live rock if you wish to make that attempt. I can assure you that if you took a culture of my rock and/or substrate that there are more than 7 strains of nitrifying bacteria like in brightwell , or Dr Tims, et al. So bottled bacteria cant compete with the diversity found from the Gulf from Tampa Bay down to Florida Bay.