Larger water changes will bring the nitrates down over time, your lfs
is correct with that. Most people don't grasp originally that a 10% water change will only take so much ot the nitrates out..in this case a 10% water change only removing 3 of the 30, giving you 27.
These 'nitrate reactors' I don't know much about, but have run media in reactors to bring other levels down. I'd assume it is similar...but in all honesty nitrates can be controlled easily through either water changes, having the right amount of rock/sand (which you do), and growth of algae in a sump setup. People have great success with both refugiums and algae turf scrubers. All of this can be easily DIY'd for a pretty low cost if you decide to go that route.