09-08-2012, 12:54 PM
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Nearly all aquariums with a medium to deep substrate (gravel and especially sand) will have anaerobic regions where oxygen levels are very low or non existent. I believe that 'black sand' only occurs if when detritus gets down to these anaerobic regions resulting in anaerobic decomposition of organic matter.
Have you stirred your sand?
Many people believe they need to stir their sand to prevent this. In fact, you really don't want to stir sand as this creates the potential of burying organic matter deep in these oxygen depleted areas which will then be decomposed by anaerobic bacteria. Anaerobic decomp results in a black foul smelling, almost putrid mess.
Note: with undisturbed sand, mulm collects on the surface of the sand and is slowly processed (decomposed) by aerobic bacteria.
You might remove the black sand this could be difficult and potentially counter productive. I think I might just leave it alone and let nature take it's course...unless you have a lot of it. If so, you may need a tear down to clean the sand.
Moving forward, do not stir sand and use care when planting and/or removing rooted plants - often better to cut and leave roots than to disturb the substrate.
Just my $.02