I struggle to get my head round the whole concept of products like these that claim to noticeably reduce nitrate. To really reduce the nitrate level as per their claim you'd have to have a lot of water being processed. But of course this cannot happen with oxygenated aquarium water. The levels of water being processed deep inside these honeycombed products must be minute, hence no noticeable reduction. It's hardly surprising that the comments so far have been negative.
And what gets me even more is, as
Rocksor
quite rightly said, is that the insides of these honeycombed structures must clog very very easily with slime and such. Basically rendering any internal nitrate filtration nigh on impossible.
Even if we could measure nitrate on a really accurate scale, and down to the nearest 0.01 ppm for instance, which the commonly used API kits don't, i still doubt you'd see much swing between using these products and not.