@Jocko
If you want to use a DSB only for nitrat-reduction you could take every sugar sized sand, you don't need the aragonit sand - that's right.
But if you want to have the other advantages like stabilizing the alkalinity, a calcium supplement and a very white coloured sand, which reflects the light to the underside of the corals, the aragonitsand is the better choice.
@Yetti
You could buy every sugar sized aragonit sand. The best one comes from the Bahamas.
Maybe you could find it at a local dealer who sells sand, stones and other building materials.
Sudad