I think the thinking now is that they are very big sinks. They will hold the heavy metals and phosphors for only so long and then they crash. I also don't see how aerating or sifting would remove heavy metals from sand or rock.
I am also not saying I subscribe to the heavy metals theory, simply pointing out that might be a reason why Fenner would recommend replacing some rock every so often. I do find logic in the DSB
being a sink. All the detritus and waste processed by the sand has an end product, not all of that end product is nitrgen, this is sunk into the DSB
, over time the sulfide layer creeps ever so slowly upwards until there are problems all over the place.