"Pszemol" writes:

I was thinking the salnity of the ocean is pretty constant all the
time...

Actually, salinity in the ocean is not constant, though the swings are
definitely smaller than in your tank. Evaporation and precipitation
can each result in rather substantial salinity changes to surface
seawater.

I wish I could speak with some authority as to how large these were...

Anyway, that's a really cool observation about your snails. I suspect
that Ron Shimek over at reefcentral.org could give you great answers
to your snail questions (since he's a scientist specializing in marine
invertebrates and a fanatic reef aquarist to boot).

If you do head over there, I'd really appreciate it if you could
repost anything you learn back here...

Regards,
Ross

-- Ross Bagley http://rossbagley.com/rba
"Security is mostly a superstition. It does not exist in nature...
Life is either a daring adventure or nothing." -- Helen Keller