| Well... I use about 1 for every 5 gallons..and I have about 1 nassarius for every two gallons. I also have 1 astrea per 5 gallons, and a handfull of ceriths... finally, I have about a gazillion collonista snails in my system that came as hitch hikers.
Basically, if there is a food source (algae for most, meaty foods as well for the nassarius) then they will thrive. If you have a new tank, with little algae, you may not be able to keep a lot of snails...
I add more snails (about a dozen or so) every year as I tend to lose some to the hermit crabs and other natural causes.
Dave
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"Fins to the left...Fins to the Right, I'm going home to play with my reef tonight..."
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Experience in aquaria, 35 years. Experience in marine, 20+ years. Experience in Reef Keeping, 6 years. Always a newbie! | 
