"Sucker fish" is a catch-all term for a few different species of algae-eating, freshwater aquarium fish with disk-shaped mouths. The term may refer to plecostomus or other members of the catfish family, or Siamese or Chinese algae eaters. Their purpose in the home aquarium is to eat excess algae, though you must feed them supplemental food, such as algae wafers and fish pellets, to insure their health.
The Hypostomus plecostomus, referred to as a "pleco" in the aquarium trade, is the quintessential sucker fish. Originating in South America, this catfish is often seen stuck by the mouth to the sides of tropical fish tanks, eating algae. There are several varieties of pleco available to the aquarium trade. Many can grow quite large, so these are not recommended for small (under 55 gallon) aquariums. These need driftwood in their tank for roughage.
Chinese Algae Eater
Gyrinocheilus aymonieri, the Chinese algae eater, is also called the Indian algae eater and the sucking loach. It hails from the tropical waters of Thailand and Indochina. It is known as a rather aggressive species, though its diet is mainly algae and other plant life. It is relatively small (under 10 inches) and is most comfortable in well-planted tanks.
Siamese Algae Eater
Crossocheilus siamensis, the Siamese algae eater, also called a flying fox, is often sold alongside "false Siamese algae eaters," which are fish in the genus Epalzeorhynchus. True Siamese algae eaters hail from Thailand and the Malay Peninsula. True Siamese algae eaters are better algae eaters than false Siamese algae eaters, and are also less aggressive. They are the only known species to consume red algae.
Also called the whiptail, Farlowella acus is a thin catfish which is a peaceful algae-eater from the Amazon basin and Venezuela. There are several varieties of twig catfish. This fish mimics its surroundings by blending in with the twigs and plants. It is not recommend for beginning aquarists. It prefers a well-planted tank to feel secure, and needs to eat driftwood for roughage.
Sometimes called a dwarf sucker fish, oto, Otocinclus vestitus, is a small algae and plant eater from the Orinoco river system in the Amazon. They are a peaceful, nocturnal species which do well in community tanks and prefer to live with other otos.