Hello and thank you for your inquiry.
It sounds like the rock is still in the curing process in your tank. Most of the time we recommend curing the rock in a separate container (usually smaller in volume) so you can do multiple water changes during the curing process. Then once the rock has fully cured it can be transfered in to the display tank where usually only a mild cycle will occur.
At this point your best plan of action is to make sure your protein skimmer is running properly and keep up with the water changes in the tank. That will hopefully help to bring the nitrate levels down for you.
There are also a few filter media's that you could try to use to help reduce the nitrates if you wanted, such as Seachem's Purigen or using a biopellet reactor on the tank.
But usually water changes for a new tank are the best way to help reduce the nitrate levels.
If there is anything else we can do for you please let us know.
Marine Depot Customer Service