I am going to remove the Rough periwinkle and replace them with one of the species where mentioned. Rough periwinkle is good in a tank, but the problem I have with them is the fact they like the shoreline more then the depth of the tank. So they would always hang in the top of the tank, creating there own problem there. As they could get killed in the top of the tank when I was moving the lid or they would fall down and not be able to get back on there foot.