Extra large trapdoor snails (Viviparus viviparus )
These snails will be between 3 and 4 cm
And can be kept with even large carp and goldfish without risk of being eaten.
 sometimes known as the river snail ,freshwater winkle, mystery snail or trapdoor snail. It is sometimes confused with the similar Chinese trapdoor snail (Bellamya chinensis). Viviparus viviparus is a large native snail species and can reach 4.5 cm this species has a very tough shell and can be kept with large carp, goldfish and other large fish that would eat most other snails species and its operculum (trapdoor) prevents it being sucked out of its shell by smaller fish such as loaches. They are great algae grazers and will graze algae from rocks, glass and plants (they wont eat live plants). As well as scavenging detritus, dead fish and plants and left over fishfood, they also filter-feed algae, bacteria and small particles from the water like mussels and are excellent at clearing green or murky water. Being a native species it can survive UK winters, but can also be kept in coldwater and tropical aquariums. Snails make up an essential part of a pond or aquarium ecosystem and would make an excellent addition to a wildlife pond.