I had 3 of those exact guys in my 90. To be honest I kind of liked them, they never bothered any of my snails and seemed to go after all sorts of detritus. They started out about 1cm across, and by the time I broke the tank down they were close to the size of the crab in the pic, one of them even started carrying around clutches of eggs!
I had more problems with hermit crabs eating snails
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