Are we just talking phosphorus, or do you include nitrogen, metals, etc,?
There you go again. LOL Tell you what 'BUY MORE SNAILS'. What you missed was the post about some corals tolerating elevated P better than others.
IF you were to say that you believed that (or allowed for the possibility of)something besides phosphorus being the culprit, I might be a lot less inclined to dispute the mechanism.
H2S, for example, seems like a very good candidate to me.
It might chronically leak and reduce the health of a reef aquarium, with more and more leaking over time as the tank and sand bed aged.
It might also acutely come out in large quantities if something (maybe something even unseen) disturbed the sand bed, causing an immediate "crash".
Algae may not be particularly harmed by H2S, while other organims (e.g., corals) may be.
H2S is one of the few things that can clearly be made in a sand bed in far higher concentrations that would occur in the water column or in systems wiuthout sand beds.
Why do you not think it a possible cause?