you can use river rock in the tank. you need to make sure you have enough live rock first.
^ageeed! Just to add to that in case you don't know the reason why we're saying this - you first want to make sure you have enough porous rock in your system. Porous rock has the required nooks and crannies that are needed for efficient biological filtration. River rock typically is flat and dense and is not as efficient. Once you have enough porous rock to cover your biological filtration needs you can add as much non-porous rock as you desire. Just make sure it is cleaned well. You don't want to introduce any nasties from the rock into your tank.

