Name: Otocinclus spp.
Origin: South America
Note: There are about two dozen similar Otocinclus species spread from the Orinoco/Amazon Basin to Southern Brazil. They will all be treated in the same page for now since most aquarium hobbyists would be unable to distinguish between them, but if you know exactly which species you own, please include it in your comment!
I find that this small algae eating catfish is best suited for a planted tank. It is quite small, and does not disturb its tankmates. It will eat all sorts of algae with exceptions of red algae and blue-green algae. In the process of removing algae, it will not harm aquatic plants. I find these fish very useful and keep several in my home aquarium.
My aunt and I have both had the Otocinclus affinis in our tanks. In both our cases, this catfish when larger has attached itself to the bodies of its tankmates. That in turn killed them. I don't know if this is a normal trait for these fish or not...
Otocinclus are happiest when they have company of their own species. Try to keep three at least, and they will reward you with their charming interactions. That would mean an aquarium of 15 gallons or more. The algae in your tank will probably never be enough to keep Otocinclus well-fed: blanched spinach, a weighted blanched "bouquet" of parsley, sliced zucchini, also blanched so it will sink, all are welcome. By the way there are many unusual species of Otocinclus slipping in with O. affinis: you may have one not quite identical to the picture above or to the others in your tank!
I've had two Otos in a fairly new 110 liter tank for just over a week. They initially stayed together for the first few days and devoured most of the algae that had developed through the cycle period. Since they are so small and there are many places among driftwood, roots, abundant live plants, and rocky caverns for these fish to hide, they are very difficult to find. Mine also will remain attached to a particular leaf or a particular place on the glass for long periods of time and then become very active. Since only one now appears at a time, it could be that the other has succumbed to his environment. This could be coincidental, however, there are absolutely no traces of his whereabouts. Perhaps he may be found in one of the filters. Nevertheless, I will add a couple more to promote a happier group. I have read that the Otocinclus affinis may loose condition very suddenly for no apparent reason and die when placed in a new tank. I have also read that they are very sensitive to medications in the tank and may die suddenly when subjected to medications as well.
My two catfish (a couple) live in a 40 liter aquarium with several other fish and two other species of catfish. They are or seem to be aggressive to all species of catfish in their breeding cycle. I recently took out my undergravel filter and put in a model on the outside. Well the two catfish disappeared for several days and I assumed they were victims of attacks. But I couldn't believe it so I waited a few more days. On about the eighth day, towards night when the lights were going out they appeared and I was delighted. But following them out were hundreds of baby catfish which quickly became meals for my other more aggressive species. So the moral is remove all totally secluded hiding spots if you don't want an excessive amount of little suckerheads.
I currently have 5 Otos in my large community tank. They do a terrific job of cleaning the tank and adapted to the new tank almost immediately. They are surprisingly active and attractive fish. The only negative experience I've ever had with these fish was when one got stuck in a powerhead and I had to pull him out by his tail. Luckily he survived.