Black substance in sandbeds
I am definately sure that any aquarists who have kept sandbeds before would have come across the statement that a black layer would be formed. Some say this is hydrogen sulphide, some say this is cyanobacteria. Actually, this black stuff is none other than FeS or Iron sulphide.
Iron sulphide itself is a brown precipitate and due to the intense colour, we often perceive it as black. All sulphides are toxic to organisms and thus, this chemical is not good for reefs.
This compound is formed by sulphate reducers which being anaerobic bacteria, usually thrive under mats of the cyanobacterium Lyngbya sp. or in any anaerobic region. Thus, in anaerobic regions of sandbeds, a "black" layer of FeS is formed.
FeS can be oxidised in the sediment by MnO2 or maganese oxide into FeOOH, S, Mn and H2O. It can also be oxidised into sulphuric acid by autotrophic or mixotrophic bacteria and will be neutralised by calcium carbonate into calcium sulphate.
All in all, the black substance is not much of a worry BUT it indicates that the sandbed has a totally anaerobic zone and this should be the main worry of aquarists