Keeping Otocinclus Catfish
Otocinclus, the dwarf suckermouth catfish, is a popular genus of algae-eating catfish. The seventeen species of Otocinclus (often called Otos) are often kept to control algae. Unlike their large cousins, the Plecos, Otos remain small and are not boisterous enough to cause damage to plants. Although they are often purchased singly for algae control, Otos are social and should not be kept in groups of less than three. They are native to South America east of the Andes mountains, in streams that drain into the Amazon, Orinoco, Paraguay/Parana rivers, and in streams that drain into the Atlantic in southeastern Brazil. They are notably absent from the Guianas.
Otos are small - they range in size from 16.5 to 43.8 mm (0.6 to 1.7 inches), not counting the tail. Females are 10-20% larger than males, and have a broader body, especially when they are in breeding condition.
Otocinclus are herbivores, feeding mainly on algae. The ideal tank has a lush growth of algae for them to feed on. Driftwood tends to support a healthy growth of algae for them to browse on. In addition, since it's difficult for them to browse the algae all the way down on a rough surface like wood, it's likely that there will always be something to eat on a piece of driftwood.
Aquariums often do not support enough algal growth to feed a school of Otos, so it may be necessary to supplement their diet with blanched spinach or zucchini. Food that has been dipped in boiling water or microwaved for a minute or two appears to be more attractive to Otos. An algae clip or a rock can be used to hold the material in place. Sinking algae wavers are also a good way to get some more protein into their diet - however, since these wavers usually have a significant amount of shrimp, they are attractive to other fish. In my main tank, the Otos rarely get at the algae wavers - the more boisterous Corydoras catfish keep the Otos from trying to get at the food.
Most Otocinclus are wild-caught (see this video, for example). Since they are vegetarian and need to eat almost constantly, mortality rates are high in transit. They may be starving by the time they get to the pet store, where they are often fed unsuitable food. Combine this with the added stress of a trip home from the pet store, and you end up with a fish that has a reputation for dying readily.
To minimise the risk, try not to buy fish until a few days after they arrive at your local pet store. Pick healthy fish - ones that are swimming around, looking for food on the gravel and glass, ones that are schooling with the other Otos in the tank. Pick ones with rounded, not sunken bellies. Choose carefully.
Gary MacDonald (see "resources" section below) recommends that you bring a "snack" for the trip home - a piece of skinned cucumber or zucchini. It's always a good idea to bring a darker bag to put your fish bag in - this minimises the stress of the trip home.
Breeding Otocinclus requires a heavily planted tank. Spawning is triggered by water changes using water that is a few degrees cooler than that temperature of the water in the tank. As with Corydoras catfish, Otos appear to use a "T-position" when breeding. Gary MacDonald's article (see "resources", below) is an excellent resource. Another account of spawning is available online at PlanetCatfish.
Species of Otocinclus
In his 1997 monograph on the species, Scott Schaefer recognised 13 species of Otocinclus:O. vestitus, O. afinis, O. flexilis, O. vittatus, O. hasemani, O. hoppei, O. mariae, O. macrospilus, O. bororo, O. caxararz,O. huaorani, O. mura, O. xakriaba. Since then, four new species have been described, Otocinclus batmani, O. cocama (the zebra Oto), O. mimulus and O. tapirape, bringing the total up to 17 species.
The definitive monograph on the genus Otocinclus was published by Scott Schaefer in 1997:
Schaefer, Scott A. 1997. The Neotropical Cascudinhos: Systematics and Biogeography of the Otocinclus Catfishes (Siluriformes: Loricariidae). Proceedings of the Academy of Natural Sciences of Philadelphia 148:1-120 (JSTOR subscription required).
Macdonald, Gary A. 2008. Oto Pilot: Buying, Keeping & Breeding Otocinclus Catfish. Tropical Fish Hobbyist, February 2008.