Doug,
Was not monitoring pH during the event. I am assuming that pH and alk were both low low for awhile preceding the event because I hadn't been monitoring my tanks very closely
This situation is now in the process of being remedied.
The dead snails were the result of the event and did not show up until the fish started dying. and in the days following. They were victims of the event, rather than contributors to it. The dead snails were removed daily.
Also, don't think the green algae beneath the cyano was dead, even though cyano covered much of it. There is an east facing window behind the tank and a healthy growth of coralline on the glass. The green algae is now growing back in a few places.