Black sand is a result of no oxygen. The bad part about it is that is allows anerobic bacteria to grow. If these got out into your water column in high numbers, it could kill your fish. Denitrification is a good thing, but make sure if you spot black sand again (try not to move you rockwork after it has settled for a while) either do not disturb it and place the rock back over it, or swirl it up AFTER you take the fish out of your tank. A good sand sifting crew, like dskidmore said, are awesome at keeping sand oxygenated. Nassarius/Llynassa snails rock.
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