It's not supposed to reduce nitrates, that are already in the tank - it should bind the nitrogenous waste before it's processed to nitrates.
Used it for more than a half of year in a skimmerless nano-tanks - not impressed, frequent changes of the filter media and more fine filter media worked better. The skimmer will be even better.
Seachem has FAQs on each product on their website and a forums with some information.
For reducing nitrates - refugium, DSB in a bucket, or denitrators.