Blue and red legged hermit crabs have an average life span of 3-5 yrs in captivity.
Most snails live about a yr some species are shorter others are longer
I've lost about a third of my crabs to various things and have had some crab procreate i have 4 blue legged hermits born in my tank but my snails have been voracious procreaters I've only got one snail left from original batch and I have about 18 or so in my tank that were created and born in the tank.
If you don't have a lot of detritus your CuC can starve to death as well it could be one of many possibilities
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 My aquarium is nothing but a smorgasbord for my cats.....
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