*** been wanting to set up a shell dwellers tank for awhile , im gonna have a few multies . what *** got so far is a 30gallon , ill be putting black sand , blue granite for rock , green grass like fake plants. *** got some shells but the opening is anywhere from 2-5" across .would these be too big ? what is the ideal size. Also is there a certain type of shell these guys wont use ? the ones i have are really cool ocean ones , they have spikes . Kind of like the picture here . Anyway any insight would be appreciated.