Ramshorn snail

The term ramshorn snail is used in the aquarium trade to describe any kind of snail whose shell is planispiral, meaning that it is a flat coil. Such shells resemble a coil of rope, or (as the name suggests) a ram's horn.

Physical description
Most of these snails are of the family Planorbidae. Their blood, like human blood–contains hemoglobin, which may lend these snails a bright reddish color. These ramshorn snails breathe air. Although most are extremely small, some may reach a size of two and a half centimeters (one inch). The shells range from translucent through various shades of brown to a dark, nearly black color. The dark color appears to originate from dietary materials not generally available in the home aquarium, although many varieties from ponds will be this dark shade.
Snails of this family are spiralled sinistrally, with the opening hole slanted downward toward the right. Large folds of skin, which serve as primitive gills may protrude out of the more open left side. The shell contains no operculum, and the snail has only one pair of tentacles.
They lay eggs in globules, which tend to be brownish in color. The globules contain about a dozen or so eggs, though it can vary. The globules are translucent, so it is possible to visually see the new snails develop in size. The newborn snails are clearish white.

Interaction with environment
Ramshorn snails generally will eat only the most delicate plants, preferring algae, uneaten fish food, and dead fish. Some varieties do particularly enjoy eating the leaves of stem plants such as cabomba and anacharis.
Some aquarium species will eat ramshorn snails. More voracious eaters include loaches (such as the clown loach or any other member of the genus botia), bettas, crayfish, and most gouramis—though many other fish will also consume snail meat. The larger apple snail will also prey upon ramshorn snails.
Good fish roommates for snails include, but are not limited to, danios, guppies, White Cloud Mountain Minnows, neon tetras, and cory catfish. All of these are non-aggressive fish that cohabitate easily with snails.
One should also be aware that red ramshorn snails are able to carry various parasitic flukes, which can be transmitted to fish, or humans. Most of these flukes require intermediate hosts, so that leaving the snails by themselves for a month or so will eliminate the disease.

Role as aquarium pest
Most ramshorn snails are considered minor aquarium pests. They may arrive in a tank as egg bundles hidden in newly acquired plants. Although their red color may make them somewhat interesting aquarium subjects, their hermaphroditic ability to breed prolifically from any two specimens can make them troublesome.
Common practices include treating plants to prevent introduction, various manual methods of control, introducing the snail eating animals listed earlier, and poisoning the snails.
Soaking the plants in various chemicals may kill off the snails and their eggs. A 10 minute bath in a solution of 20 parts water to 1 part chlorine bleach has been suggested for hardier plants, followed by soaking in water containing a dechlorinating agent. A more gentle treatment calls for 5-10 tablespoons of alum to 1 gallon of water for 2-3 days. A safer alternative may be placing the plants in a quarantine tank, and adding snail poisons to that tank rather than the main show tank.
Manual methods include baiting the snails with lettuce, cucumber slices, or food pellets. These may be left out in the open, and removed with their snails, or kept in some container, such as a film canister weighed down with a pebble, and containing holes drilled in it. Crushing the snails by hand as they appear can also effectively limit their population; most ramshorn shells are fragile enough that this is quite easily done.
Snail poisons are generally considered to be a last resort, as most of them are copper based and potentially toxic to plants and fish. Even new safer chemicals that do not harm the other aquarium inhabitants may cause damage if large numbers of dead snails are allowed to decompose. For this reason, it is best to reduce the snail population by other means as much as possible before resorting to poisons, and to do frequent water changes afterwards. Some also recommend adding ammonium protection to the tank. Zeolite chips, and various liquid products such as amquel may help in this area.

Eponymous snails
Totally unrelated to these ramshorn snails is the species Marisa cornuarietis, which is often sold at pet stores under the name Columbian Ramshorn or Giant Ramshorn snail. This species is actually a type of apple snail. It is distinguished by having two pairs of tentacles, an operculum, and a siphon on the left side. Its shell is yellowish, with brown stripes running the length of the shell. These apple snails lay gelatinous masses of eggs on submerged portions of plants. Apple snails can grow to up to four centimeters in size. They generally will not become a pest, although they can consume large amounts of plant matter.

http://en.wikipedia.org/wiki/Ramshorn_snail