Patty,

I do not know the proper chemical terms but you problem is a common one to
anyone who has treated a marine aquarium with copper.  The copper in
solution combines with coral (or in a tank without coral, limestone rocks or
anything similar).  This bound up copper is released any time the Ph of the
tank drops or becomes more acidic.  To really get rid of the residual copper
you should replace all of the rock and any ornaments you have in the tank.
After having said all of that, I would expect the algae to have a tough time
with the copper also and it may be that it absorbs the copper and this is
what double doses your snail.

Tom Eichhorst in New Mexico, USA