One Madagascan cichlid of the genus Paretroplus can bring down a snail population to almost nil in 24-36 hours, in nature snails are their prefered food. They crush small snails, and have 2 can opener like teeth to extract the flesh from large snails.
above is Paretroplus maculatus, below P menerambo (menerambo gets too large for a 55), maculatus would be a better choice, as would be P kieneri.
kieneri below
I was quite surprised at how well a single male pea puffer has kept a mts population to a perfect minimum in my 25g planted, where I never see them but know they're still in there to sustain himself.
I haven't tried this myself but I read that you could rubberband a lettuce leaf to a rock overnight then remove it in the morning when all the snails are munching on it and do this everyday till they're gone.