If your snails are dying its definitely dinoflagellates. Siphon as much as you can and try reduced light levels. I wouldn't do more than a couple of days if you have any prized corals, either that or get someone to babysit them (dip them before they go into any other tank). Then black out the tank for longer than a couple of days. These things are horrible to get rid of, they are persistent and resistant to most treatments. I battled them for a year and the only way I finally got rid of them was to remove the sandbed, powerwash the rock and completely restart the system (worst case scenario IMO).
Things I tried that you may have success with:
increased pH
increased alkalinity
increased skimming
reduced photoperiod
no photoperiod (blankets over the tank too)
adding a refugium
dripping kalk
And some links to read:
http://www.canreef.com/vbulletin/sho...inoflagellates
Good luck.
