cmon, at a certain point , you have to admit with some coral colonies, it's own sheer size will kill off the oldest and lowest polyps. granted, it will differ from species to species, and is probably more true and evident with giant brain coral heads. my own sps i see what looks like the beginning evidence of this. small 2-3" colonies are great on top, but lower branches that are in the shade are almost bleached in the darkest areas. i can only imagine that in time, as the coral grows , these will be the first to die off, but not necessarilly leading to STN / RTN, but rather just natural life cycle.