Iron Bacteria - A non-health threatening but nuisance organism that thrives on iron as its food source.  It can colorize in water systems by showing up intermittently as an undefined reddish-brown mass.  It usually appears as a slick film on the surface only of water in an untreated (no Tidy bowl and such) toilet tank.  It severely effects water quality and the operation of water treatment equipment if left unaddressed.  Presence is expressed as positive or negative.  Please note - it may be necessary to double check sites with marginal samples.