Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
Looks to me like your sand substrate is a fairly thin layer in most areas of the tank bottom, so even if you pressed some of your root tab ferts down to the bottom of the tank, they're still not very far down, and the root tabs while they disintegrate are leaching their dark color to the sand.
Nothing of a problem if that's the case (which I truly believe it is- cause it's happened to me - with my white PFS), so don't worry about it.
That doesn't explain the rotting plant root ends though - perhaps those plants are just not surviving very well for whatever reason.