First on the list is probably the most hated snail type you can find, the pond snail. These snails have conical, “football” shaped shells, and are usually dark brown. These snails are hermaphrodites, which means each snail has both male and female reproductive organs. So long as you have two, the snails can breed with one another. These snails, in addition to being very quick reproducers, eat most vegetation. This makes them a bane, rather than a boon, to most planted aquariums. Most often they are introduced to aquariums when eggs or live snails are on plants that are purchased for an aquarium. Treating the plants to disinfect/de-snail before adding them to the tank is the best way to avoid a snail epidemic.
The second very common snail that can be introduced to aquariums by hitching a ride on plants is the ramshorn snail. This snail is often dark brown like pond snails, but may be mottled or striped as well. It is shaped like a flat disk, spiraling in on itself, in the shape of a ram’s horn, and thus the name. These snails are much less harmful to plants than the pond snails, but some species will eat certain live plants. They also are hermaphroditic, and prodigious breeders.
Malaysian trumpet snails are a common favorite among aquarium keepers. Like the previous two types, trumpet snails are hermaphroditic and can reproduce explosively. One advantage to trumpet snails is that they don’t just skim over surfaces, they actually burrow in the substrate of your tank. This, combined with the fact that they are nocturnal, makes them hard to detect. These snails are similar in shape to the pond snails, but significantly longer in relation to their girth. They can grow to about 1.5 inches, so they are not massive snails. These snails prefer dead plant matter to live plants, and spend most of their time burrowing into the gravel looking for food. This characteristic makes them very effective at circulating the gravel and cleaning out gunk, but they can make keeping plants planted in the substrate difficult to keep in place.
The final snail type on the list is the apple snail. Apple snails are not a single species, though often the largest apple snails are the only ones commonly labeled as such, with the smaller species called mystery snails. All apple snails have the unique feature of having water-breathing gills on one side of their body and an air-breathing lung on the other. They have a snorkel-like appendage attached to the lung that allows them to breath air while remaining under the water. Any snail with such a feature is by definition an apple snail. Some of the largest apple snails can devour the plants in an aquarium, but the smaller ones commonly sold as mystery snails are usually harmless to your plants. Another feature of apple snails that is different from the previously covered species is that they are not hermaphroditic; there are distinct male and female members of the group. This means if you intend to breed your apple snails, you will need to purchase several to make sure you get at least one of each sex. You also need to use caution with keeping apple snails; since they can breathe air, they have a habit of leaving their tanks if any openings in the lid are large enough for them to escape. They especially attempt this when they are looking for a place to lay their eggs, as they do not lay their eggs in water. If the snail exits the tank, he will try to go down to get back to water, but obviously if he is on the other side of the glass when he tries to return, he won’t find water, he will glide along your floor until he gets too dry and closes up. The best thing to do to prevent this is to make sure no opening as large as he is exist in the lid of your tank. These are the largest of the snail types covered here, and are the most interesting in my opinion. Some species can grow to be half a foot across! All are quite round in shape, almost spherical, and are available in a variety of colors.
No matter what snail species you have, if you want to keep them healthy you will need to provide hard water. They grow their shells with calcium and carbonate (GH and KH) so a limestone or crushed coral substrate or filter media is a good thing to have if you want to keep snails. Also, when your snails die you should leave the shells in the aquarium so they can re-dissolve and nourish other snails.
So what do you do if you have snails and don’t want them? If you want to reduce the population of snails, you can place a spinach or lettuce leaf in the tank overnight, and collect the snails that gather on it off in the morning. You can also get a fish that eats snails, such as a clown loach. These won’t completely eliminate the problem, but especially a loach will keep the population very well reduced.