Jake
It is a canister filter with a forced flow under pressure. This drives the water more into the media oxygenating it, thus reducing the area for faculative denitrifying bacteria to live, bacteria that do not like oxygen. It is these bacteria that take Nitrate and convert it in to Nitrogen gas, which escapes into the atmosphere, leaving you with little or no nitrates. It is not that it won't work it just wont' be as effective. Slowing down the flow rate may help.
Dan
if you are worried about high nitrates you can use a ozone device to help reduce them. If you go that route you might want to consider a monitor and controller.
But Dan an ozonizer can also produce nitrates. Some molecules and N* ions pick up a O from the O3, through oxidation. ie, NO2- + O3---> NO3-. But in general I know what you mean.