Otocinclus catfish (Otocinclus affinis)

Otocinclus catfish (Otocinclus affinis) are commonly called “Oto Catfish” or “otos” is a small and not bright fish – however, it’s one of the most efficient algae eaters in a tank.

Otocinclus together with Caridina japonica shrimp are able to keep you tank and all plants in it clean and tidy.





Inhabitance in the wild

Otos mainly inhabits in small or middle-sized rivers with moderate flow. Usually you may encounter this fish in streamside vegetation, especially in the grass among the water plants with large number of small leaves.

You may also see this catfish in some open water mass. There Oto catfish swims in shoals above sandy bottom areas filled with tree roots.

Otos are spread from Columbia to North Argentina. The fish used to be found in the Andes Eastern part. One Oto catfish species has rather restricted habitat in Peru, Brazil and Paraguay, while the other its species can be seen everywhere along the Amazon and Orinoco river basin headwaters.


Otocinclus body is elongated and flattened from sides covered with wide bone scales (the whole body, except the fish abdomen). The fish has large eyes; its mouth is cupule-shaped. Due to its intestinal respiration Oto catfish can breathe with outer air.

Otos body coloring has some yellowish tint and there is a wide black-brown stripe along the fish body. Otocinclus fins are transparent and colorless with some greenish tint.

Otocinclus female is fatter and larger than the male fish. Otocinclus body length may be up to 5 cm long (2 inches).

Otos lifespan is up to 5 years in a tank.

Nowadays Otocinclus species numbers about 16 or 17 different kinds, which are sold in pet shops like one Otocinclus species. Otocinclus Affinis and Otocinclus Vittatus are more commonly seen. Despite the fact that high percentage of Otocinclus on the market is represented by O. affinis or other species falsely considered to be O. Affinis, we must say that it is quite seldom encountered in the wild nowadays. Maybe this was caused by the changes of its native habitat and land use conditions there, in South-East Brazil.

Difficulties in keeping

Despite its small size O. Affinis is quite a demanding in care fish. It requires ideally clean water with stable parameters. Therefore, it’s not recommended for the beginners.

However, Oto catfish is second to none among any other fishes as for efficient tank cleaning from algae.

The fish does it all day long – it looks for algae and it doesn’t harm even the smallest and the most tender tank plant leaves, such as Hemianthus callitrichoides Cuba, Glossostigma elatinoides and even Java Moss.

Keeping in a tank

It’s better to keep Otocinclus affinis in groups of 5-10 species. Takashi Amano advises to have 100 liters of tank capacity (26,42 gallons) for about 20 fish when starting Nature Aquarium and about 2-3 O. affinis species after the ecosystem settles down.

If the tank isn’t that thickly planted, then 10-12 Otocinclus affinis species for 100 liters of tank capacity (26,42 gallons) is enough. In case of 30 liters capacity tank (7,93 gallons) 4 Oto catfish will do just fine.

Care is the same for all Otocinclus affinis species.

It’s partly caused by the fish small size and its intolerability to polluted and dirty water. The latter is the main reason of the fish good health and fast growth in tanks with low plants density and perfect quality of tank water.

You’ll need some strong water filters capable to renew the whole amount of tank water at least 3 times in an hour. Such filters have to insure absence of ammonia or nitrites and decrease the amount of nitrates up to 0-20 ppm. For constant renew of a tank system and to keep comfortable tank conditions it’s necessary to renew about 25-30% of the tank water every week.

Chemical composition of the tank water isn’t very important, the main thing is that the water should be clean and fresh water, the temperature should be 22-28 C (71,6-82,4 F), and pH balance of the water has to be neutral or slightly acidic.

Once you create such tank conditions and your Oto catfish will feel itself at home. In the wild Otocinclus affinis prefers to stay in large shoals, so if you’d like to keep this fish in your tank, then buy at least 6 species at once, or even more, if you can afford it considering your tank capacity and algal covering in it.

The healthy fish is very active during the day. It tirelessly eats soft small algae from any surfaces in a tank.

Add some preliminary soaked oak tree roots, round stones and tough tank plants, since Oto catfish teeth aren’t strong enough to gnaw strong scapes of the tank plants, so if the fish is out of all the supply of fresh algae, it’ll need some additional feed.

You can create a model of the catfish natural habitat by putting plants with lots of leaves and preliminary soaked tree branches into a tank. Using of sandy bottom substrate will become a good ground for the tank plants and the fish waste products will stay on the substrate surface till the tank filter sucks them in.

A good quality filter with moderate or intense water turnover will keep the tank water clean and saturate it with oxygen.

Scientific NameOtocinclus affinis
Common NameOtos, Oto Catfish, Otto Cats, Ottos, Dwarf Suckermouths, Dwarf Oto, Dwarf Ottos, Dwarf Suckers, Algae Scrapers, Macrotocinclus affinis and Otocinclus affinis
Tank size100 liters (26,42 gallons) and more
DietOmnivorous bottom feeder
Temperature22-28 C (71,6-82,4 F)
Size0,5 inches (1 cm)


The stomach of Oto catfish species caught in the wild is mostly filled with algae and other nutritive organic substances. In a tank the fish will eagerly eat algae from stones, tank plant leaves and glass walls.

However, Otos doesn’t depend on algae as on the main source of its food in the tank, therefore the fish should be fed with various special vegetable food, some feed with spirulina, flakes, frozen blood worm and gammarid.

Fresh steamed vegetables, such as lettuce, spinach, squash and green pea are perfect supplements to the fish diet.

These additives can be given to the fish once a week for the night, but the food leftovers should be removed from the tank in 2 days, not longer. New squash will be the best addition to the fish diet.

You should boil them for about 2 minutes or put them into microwave oven in a plate with water for 30 seconds, after that fix them to some stone in a tank, so Otos can eat them.

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Since Otocinclus affinis are peaceful, calm, they don’t grow to become a large fish and don’t harm anyone, they are perfect algae eaters, – they are ideal tank mates to any peaceful fish and they are welcome in any tank (for example: Serpae Tetra, Glowlight tetra, black skirt tetra, Dwarf gourami, Betta fish).

The only tank that won’t do for Otos, it’s a tank with large cichlids (Flower Horn, Oscar fish, Green terror) or catfish (Sailfin pleco).

Sex differences

Oto catfish exhibits rather pronounced sexual dimorphism – Otocinclus affinis female fish becomes larger and wider after they become reproductive. Otos male fish has genital papilla and rows of modified denticles (odontoids).

All of these are located on the bottom side of the fish tail-stem, where its body becomes a tail which is specifically twisted. The role of this organ is still not clear enough, however some specialists suppose that it participates in the fish copulation and it is meant for the fish male orientation during this process when he embraces the female fish head with his body.


Otocinclus affinis breeding is qualified as a difficult process. Though the fish was bred in captivity, still it happens occasionally. Although it’s easy to see between the fish male and female, to provide successful breeding it’s better to keep a group of Otocinclus affinis.

To stimulate Otocinclus affinis spawning you should feed the fish with high protein feed, which consists of live feed, several times a day. Rather long courtship period that consists of the fish fights and getting the spawning spot ready precedes Oto catfish spawning.