[APD] Is this caused by anaerobic bacteria?

["Seweryn Bialasiewicz" <s_bialasiewicz at hotmail_com>]

Hello all,
I think this black stuff is the product of anaerobic bacterial action. Can 
anyone confirm my hunch?
Pic is here, but there is a UQ front page that you have to click the 
"proceed" button to get through to the pic!

www.uq.edu.au/~uqsbiala/smallgravel.jpg

BTW - is this a sign of poor substrate health? Will the anaerobic products 
be detrimental to the plant roots or in any other way?
Cheers!

- Seweryn

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