If you've got sand and liverock in your tank, you should have plenty of bacteria in that tank to keep you going. Get some live rock rubble, or even some baserock rubble. (it eventually becomes live) This will become home to ANAEROBIC bacteria, which will actually help to eat up your nitrates. bioballs just house AEROBIC bacteria that convert ammonia into nitrate, in effect, creating more nitrate.
I've also heard good things about some of the higher-end ceramic biomedia. I bought 1lr of seachem matrix months ago and placed it in my sump along with some rubble. I haven't seen nitrates in my system in ages, and I actually have lower than the recommended amount of liverock in my tank. (though that'll soon by fixed.)