Snails need decent water just as much as fish and other invertebrates do. Pond snails are not as sensitive to lower temperatures as much as tropical fish are, so a heater is not essential if your home is heated. A decent filtration system is needed for the plants and the snails to really prosper, although it is possible to balance plant growth to keep the snail wastes processed, just as it is possible with fish present. Water circulation is needed for the plants as well as the snails. If you have decided to never have fish in the tank and just want to keep the plants and snails healthy, you can get away with using a power head to provide the water circulation.