mysterious snail deaths

[nlreefguy]

I don't really understand this problem - snails just don't seem to live in my tank. I have added nasarrius, cerith, astreas, turbos - all with the same result: the vast majority of them just never do eat and just die where they laid when introduced or very shortly after. They are all acclimated well over about an hour or so in a near-drip fashion. The water parameters are all great (ph 8.2, nitrates and phos 0 or undetectable, alk 9, calcium 400 or greater, mag 1350 range, no ammonium or nitrites, temp 79 to 81, salinity 1.025 according to my refractometer). Algae is growing on the glass, rocks, and sand (enough to sustain a small population of snails) and i've only ever added them as the need arose, so it's not like I'm stocking 50 snails and then wondering why some don't live. Only small numbers have ever been introduced at once... and it's not a predator or parasite, these snails just languish and die after introduction and then are consumed by scavengers. Any ideas???