Here's a reply to a similar question. This was posted by
Anthony Calfo on the WetWebMedia forums:
as you might guess, they are not growing from thin air - or water as it were - but rather, they are filter feeders that are flourishing because of excess nutrients. Better nutrient export (or limiting import) will easy check these creatures and force them to wane. If skimmer performance has waned (less than several dark cups of skimmate weekly minimum), or if the water change schedule has been too modest/small all along and caught up with you (20% per month or less), or if feeding habits are sloppy like mine . like thawing frozen foods but not decanting the thawed pack juice which is "rocket fuel" for growing nuisance algae, sponges or Vermetids in this case. Some possibilities to consider. But I can reassure you my friend... control the nutrients and you will control their growth. There is no "reef safe predator" on these snails for the aquarium. Anthony 
