any media, once colonized by bacteria will become beneficial...
the hype around southdown is that it is a fine aragonite sand that was available at HomeDepot for a FRACTION of the cost of what sugar/oolitic aragonite sand was being sold for at your Local Fish store.
it is also the perfect medium for a Deep Sand Bed, which had reached its highest popularity at the same time this stuff was available.
Hope that helps, if you want more info... just do a search on "southdown".
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