Yeah, we need a time frame here.
For example, if your seastars, urchins, snails, etc died within several days, then I would say acclimation (how do you typically acclimate?). Seastars, keep in mind, are INCREDIBLY sensitive and need long acclimations...what color and kind were you trying? Your tank may simply be too young for many.
If everything dies...including that you don't see much in the way of hitch hikers like copepods, amphipods, worms, etc. then I would consider some sort of heavy metal situation (Don't have any brass fittings, stainless clamps, etc?). Also check the water coming from the RO for copper.
When you mention the snails dying, do empty shells just turn up, or do you see them on their backs, almost paralyzed?