Originally posted by jackdp
Southdown wouldn't have any major effect on pH.
Normally this is true. If you add dry aragonite sand (the Southdown) to tank water it WILL!!!!! precipitate Ca, Mg, and CO3 out of solution. this will lower your ALK to the point where there is almost no PH buffering. This is what is affecting the snails and is probably stressing your fish.
I agree that you shouldn't remove the sand now, the damage is done. I suggested water changes not to clear up the water, but to restore your Ca, Mg and ALK levels.