Here is some info from Dr. Ron
"Conchs, or members of the species in the gastropod group called the Family Strombidae, are strictly herbivorous. Within this family are several genera, including Strombus (1,2,3,4,5) (true conchs), Lambis (1,2,3) (spider conchs), Tibia (1,2) (tibias), Varicospina (beak shells) and Terebellum (terebellum), that differ significantly in shell form and structure. Their internal anatomies, however, are similar. Unfortunately, many of these animals, particularly those in the genus Strombus, have a basic shell shape that most people think is more representative of the carnivorous whelks. This type of shell is tapered at both ends with the front end elongated out into a calcareous spout or siphon. In some cases, ignorant or malicious dealers have sold some of the predatory whelks as various conchs. There is, however, a surefire way of discerning whether you have a real conch. When the animal extends from its shell, look closely at its eyes. In all of the animals in this group, the eyes are large and evident, located at the end of a long stalk, and they possess an evident eyeball with a visible pupil. They are visually oriented animals and will watch you. In the predatory whelks the eyes are typically just a small black dot with no discernable structure and are located at the base of the tentacles coming off the top of the head. The conchs have a long and evident proboscis which they use to bite off chunks of algae. In a very real sense, they are not rasping grazers but biting eaters; however, they are superb at eating algae.
All strombids are animals of the sand and have a small muscular foot which is not adapted to crawling on rocks. They actually move by lurching or jumping, in what has been called "saltutory locomotion" by the researchers who have studied it. Saltutory is a $5.00 word for "jumping." Never let it be said that invertebrate zoologists would use a simple word when an odd or complex one was available. This mode of locomotion is good for sand substrata, but is inefficient and largely ineffective when moving on rocks, so the snails tend to stay on the sand once they have reached a shell length of an inch or so.
A good-sized sand bed is needed to keep these animals successfully. Generally, as a rule of thumb, there should be about one to two square feet of open, algae covered sand per inch of snail shell length. Conchs will move around the bases of the rocks and remove algae from quite high up on the rocks with their amazing proboscides. They are interesting animals, and a valuable addition to a tank that can support them.
Some of them, such as the Queen Conch, Strombus gigas, grow to quite large sizes. Adult queen conchs may reach 16 inches in length and are really too large for any home aquarium. The ones for sale in the aquarium hobby are aquacultured and are small juvenile individuals, often about one or two inches long. However, if given enough algae, they do grow well; I have had one that added an inch of shell length per month for about a year. In doing so, they will get too large for a small tank and will then starve to death. Other smaller conchs are equally good at grazing on algae, and should be purchased instead of Strombus gigas.
Several of the smaller conchs, such as the Florida fighting conch, Strombus alatus, have the habit of disappearing under the sand for extended periods. In some cases, they actually will move along under the surface of the sediments, eating algae; in other cases, they will stay in one place but use their proboscis to forage in the sediments up to a couple of inches from the animal. Although they appear to be invisible, they often extend their eyes to or slightly above the sediment's surface, and careful observers can find them by searching the sediment for the watchful snail."