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The term ramshorn snail or ram's horn snail is used in two different ways. In the aquarium trade it is used to describe various kinds of freshwater snails whose shells are planispiral, meaning that the shell is a flat coil. Such shells resemble a coil of rope, or (as the name suggests) a ram's horn. In a more general natural history context, the term "ramshorn snails" is used more precisely to mean those aquatic pulmonate gastropod mollusks in the family Planorbidae that have planispiral coiled shells.
Ramshorn snails have been bred for the aquarium trade and various color forms have been selected. The two species commonly found in aquariums are Planorbella duryi and Planorbarius corneus, both in the family Planorbidae. One species (Columbia ramshorn, Marisa cornuarietis) is from a totally different family, the Ampullariidae.
Ramshorn snails can sometimes become a nuisance in an aquarium where they have access to an unlimited food supply. However, in properly managed aquaria that are free of excess detritus, they are rarely prolific enough to become a nuisance to the home aquarist.