Snails Dieing

I am new to the hobby have a 75 gallon tank everything seems to thrive but snails seem to become paralysed and die.  Cant even keep big turbos alive and all normal water tests are good.

You have come to the right place for help with all your reef aquarium questions and troubles. In order to help you, some of the things we need to know are:
How was the tank set up
Can you show us a pic
Are there other inverts (coral are inverts too) that are able to survive? Hermit crabs maybe?

I hope that you are removing dead snails as soon as possible, because a bad algae bloom is sure to follow unless this problem can be resolved soon.
Phosphate (PO4) would be the most important thing to check for. Snails die when PO4 is too high. PO4 is removed with either of two chemical media; Activated Alumina (AA) or Granulated Ferric Oxide (GFO)

*** But before we point to PO4 as the culprit, what was the LR and LS that was used in this tank? Where did it come from If it was LR or sand that was allowed to die and then was used in this tank, that would be the culprit. LR and LS have living organisms way deep inside. If LR and LS are not treated the same as a fish or coral would be treated, they die and cause pollution. In this case PO4 may not be too high but other pollution not detectable by the usual testing would be the thing to address. Look in the WMAS Reefkeeping Tips below in my sig line for the Tip titled "Can I use the dry rock and sand that came with this aquarium? The instructions of how to deal with it are there.

My gut instinct is to have a sample of water checked out for phosphates at your local fish store

I believe that the starvation that occurs during transport is one of the major reasons that snails get sick and die in our tanks. Since each type of snail is collected in different areas of the world, each one has a different transport and holding experience. A collection of snails that came together at an intermediate holding point could have been mishandled. Holding tanks do not typically have algae growth sufficient to feed thousands of snails. Many may have died at that point and were discarded. The remainder were just barely hanging on, surviving just long enough to make it to our tank.

Though everyone, from diver to LFS does their best to keep animals alive during the time they have them, there may be insufficient regard to providing all that the organism needs to be strong enough to survive all the way to our tank. I believe this is one of the major arguments against the hobby.

One thing I have noticed is that snails coming into my tanks where there is plenty of easy soft algae to feed on and then ample places to hide for the day, seem to recover and get stronger day by day.
Astrea snails and Pacific Turbo Snails need careful acclimation, depending on the condition of the water they arrived in. Transported Astrea Snails in their weakened condition cannot right themselves, so the hobbyist needs to carefully flip them right side up.
Margarita and Cerith snails seem to handle transport and bad conditions somewhat better in my experience.

That's my 2 cents