Those are the common European garden snail, Cornu aspersum (Helicidae). It is the main species used as food (scargot), but other species are also used. It has been introduced to many places worldwide, including the USA. They are an impportant agricultural pest. They obviously are remarkably variable in coloration, including melanistic and near white, but most are variations of the concentric broad stripes with smaller addional markings. The fish should love them ((if offered cracked, or large fish who can crack them).