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Related Catfish FAQs:  Otocinclus, Loricariids/more than PlecosLoricariid Identification, Loricariid Behavior, Loricariid Compatibility, Loricariid Selection, Loricariid Systems, Loricariid Feeding, Loricariid Reproduction, Loricariid Disease, Catfish: Identification, Behavior, Compatibility, Selection, Systems, Feeding, Disease, Reproduction Algae Eaters

Related Articles: Suckermouth South American Catfishes, the Loricariidae, Siamese Algae Eaters, Algae Control in Freshwater Aquariums by Bob Fenner, Dealing With Algae in Freshwater Aquaria by Neale Monks, (some) Algae (in moderation) Can Be Your Friend, ppt presentation, Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, by Bob Fenner, 

/The Aquarium Gardener Series, /A Diversity of Aquatic Life

The Ideal Algae Eater? The Littlest South American Suckermouth Catfishes, Genus Otocinclus

By Bob Fenner

Otocinclus doing their thing

Of the thirty four living families of catfishes, a few stand out as aquarium favorites; the armored cats of the Callichthyidae (e.g. Corydoras, Diadema), the long-whiskered pimelodids, the gorgeous Synodontis of the family Mochokidae, and the "plecostomus" family of South American Suckermouth catfishes, the Loricariidae. The Loricariids are often called upon to do "double duty" as aquarium specimens; to look nice and be a/the clean up crew removing algae and other waste. This way too often doesn't work out as planned.

There are some principal misunderstandings regarding the Loricariids; as basic as which species are which, what water quality they enjoy/will tolerate, and providing them adequate to appropriate foods. Many species are in taxonomic limbo and numerous (at least two to three hundred) are yet to be scientifically described. Some are huge, attaining more than two feet in length, and quite aggressive/territorial; fighting back any and all tankmates. Others, like the genus Otocinclus we'll be covering here, are small (a couple of inches maximum) and retiring.

In "browsing" the electronic bulletin boards regarding freshwater fishes, live plants and aquariums I've noticed a great deal of interest in these "little suckers"; and several references to lack of success in keeping them. The "formula" for Otocinclus success is actually not mysterious nor any secret at all. After handling hundreds of boxes of imports of these fishes, and observing attempts at their commercial production in the Far East, I've come to the this parsimonious conclusion. As is often the case with all livestock husbandry, the principal concerns are merely selection, proper water quality, habitat and feeding.

Classification: Taxonomy, Relation With Other Groups

The catfishes are a huge assemblage, some 34 families, 412 genera and 2,405 described species, making up the Order Siluriformes. Of these, the Suckermouth armored catfishes, family Loricariidae, collectively termed "plecostomus" or "Plecos" by hobbyists, are an enormous group themselves. The largest family of catfishes, with about 80 genera and 600 known species, some systematists subdivide the Loricariids into five subfamilies based on head shielding characters (See Nelson, 1994, Burgess, 1989).

You know these fishes; dorso-ventrally (top to bottom) flattened, with underslung sucking mouths surrounding fleshy lips bearing reduced barbels, and spiny body armor ("loricar" means armored). Of special note are Loricariid fish eyes; those omega-shaped pupils dilate and contract in response to varying light. Most advanced fishes eyes have fixed pupils.

The genus Otocinclus has at least twenty valid species to date with several more yet to be scientifically described.

A few quick notes for comparison's sake re 'other' algae eaters. The most widely sold in the west, the Hillstream fish of Southeast Asia (family Gyrinocheilidae), Gyrinocheilus aymonieri, is not even close in utility compared with Otocinclus. The so-called Chinese algae eater only does a passable clean up job when small; becoming a plant (and fish) eater with age/growth. The Siamese algae eaters, genus Crossocheilus vie for first place however. We will deal with these minnow-like fishes (family Cyprinidae) in a separate piece.

Prominent Species of Otocinclus Useful to Aquarists:

The classification, or more accurately, identification of these catfishes in the pet-fish industry is often erroneous, but should not worry the hobbyist unduly. All members of the genus are about equally hardy and industrious. To-the-species taxonomy is based on scale counts, teeth and body armor arrangement and relative body measurements. All under two inches in length.

Otocinclus affinis Steindachner 1877, the Golden Otocinclus, is probably the most commonly imported species (out of southeastern Brazil, near Rio de Janiero). It is brownish grey overall with a darker lateral body band. Not the hardiest oto, this species prefers soft, acidic water. 

Bigger PIX: The images in this table are linked to large (desktop size) copies. Click on "framed" images to go to the larger size.

Otocinclus vestitusCope, is also commonly available, coming from a wide range of streams in Peru, Brazil and Colombia. Often confused and mislabeled as O. affinus, though quite differently marked and geographically distinct.

Otocinclus flexilisCope, the imitator or peppered Suckermouth occurs in the same habitats (rivers of Brazil and Argentina) as the marbled Corydoras, C. paleatus; both are similarly marbled in appearance.

O. mariae, O. vittatus, O. maculicauda, O. notatus, O. leucofrenatus and a few other "miscellaneous" Otos occasionally make it into the trade intentionally or as "contaminants" with other species' shipments (often not even catfishes). Keep your eyes open for them.

Otocinclus species, the "Tiger" Otocinclus. An example of occasional offerings. These ones being spawned in good numbers in the orient for the pet-fish interest. Images shot at the 2001 Aquarama Show in Singapore.

Just a mention of the closely related genera Parotocinclus and Hypoptopoma. Though rarely imported, some of these are beautifully eye-catching, and just as suitable as Otocinclus.

Natural and Introduced Range

The Loricariids are entirely freshwater and of the New World, Panama to most of South America. Waters of low elevation to more than 3,000 meters, still to swift-flowing. Otocinclus species themselves are more confined to low-lying rivers and streams of the immense Amazon basin, principally the Rio Grande do Sul and La Plata regions (so far as described).

Selection: General to Specific

Pick out initially healthy stock; this is a crucial and difficult task, and no, I have no crystal ball. Some shipments of Otos are almost 100% DOA, others the opposite. Sometimes "bad" (more than 50% lost at the get go) imports suffer very little consequent loss, whereas others are all dead for no apparent reason within a few days. A few generalizations are of service:

1) Look closely at the whole batch; every individual in the place. How do they seem? Are they full bodied; that is, not skinny? Are the fins intact? Not frayed or whitish? All breathing? Alive at least? Good, go to 2).

2) Look for good color, olive brown, black and white where it should be for the species. See any with grayish white, or bloody marks, especially at the paired fin origins and belly area? If their skin is damaged, leave them all.

3) "How many do you want?" Pick out the active individuals, those cleaning up, moving around. Omit the skulking, immotile ones having "private parties" in tank corners.

4) How long have they been there? If they're going to die, they generally do so within a few days... from "ammonia" (or other metabolite poisoning), low oxygen, lack of feeding, poor handling in general. Wait for them to "plump up" and stabilize.

X) For you wholesalers, retailers who are receiving and holding these animals for a while (as opposed to transhippers, break-packers/jobbers): A) do utilize acidified water and Methylene blue (for oxygen carrying ability and anti-microbial effect) in acclimation (Fenner gratuitous self-citations below). B) And make sure they're provided with some driftwood and other purposeful food. The best outfits I've seen keep some microwaved/blanched zucchini, nutritious food blocks, and defrosted frozen bloodworms in their system at all times. C) Deworm them en masse with medicine (hobbyists can use Tetra's medicated food for parasites) applied to the food, before sending them on.

Environmental: Conditions


If you'll take the time to seek out and acclimate healthy Otos and do nothing more than provide them with an appropriate habitat, dark spaces, live plants, and clean moving water of high oxygen tension, you will find them to be extremely hardy.

As a corollary, don't be too fastidious about scrubbing the system sterile; instead just clean the front or viewing panel(s) leaving some algal growth for your army of Otos.

Otocinclusare not as nocturnal as their larger sucker mouth brethren, but do appreciate cover to get out of the light and into hiding. Ideally this would be a combination of plantings, submerged wood, and other decor.


The Otos display a wide tolerance to given standard aquarium-utilized parameters, living well in soft to hard (zip to 400+ ppm total hardness), acidic to alkaline (5.5 to 8.0) waters. They do display an objection to high concentrations of organic metabolites however. You should have no detectable ammonia, nitrite, and low nitrate (less than 10 ppm) levels. The steps to maintaining these criteria are simple; under crowd, underfeed, properly filter, and do regular maintenance (in particular water changes) regularly. Poor water quality manifests itself in death due to apparent bacterial infection.

Temperature range is wide enough, 72-82 F. (21-27 C.). If summer temperatures peg out much higher, cut back on your lighting and increase/add aeration.


Should be vigorous and complete; move all the water around and through the mechanical, biological plus possibly chemical filter media a good twice plus per hour. As long as it does not disturb your plants, you can't have too much water turbulence.

Additionally, weekly water changes of 25% are recommended to keep metabolites low and the system "dynamically stable" (love those oxymorons!).


Though Otos are technically armored cats, they should be handled gingerly, as little as possible, and only then with fine meshed nets. Avoid touching them by hand, wiping off their protective mucus and possibly getting spined by their small but spiny dorsal and pectoral fin spines. Instead, invert the net and cats into the intended system's water and let them extricate themselves. Commercial operations do well to fashion shallow draft, rectangular nets utilizing fiberglass screen door netting for these and other similarly spiny livestock.

Behavior: Territoriality

Some Loricariids are decidedly agonistic toward conspecifics, the clown "Plecos" (Peckoltia), and royal types (Panaque) come to mind. Such is not the case with Otocinclus; they get along with their own, other members of the genus, in fact with most everybody. There are citations that call attention to "riding" and parasitic sucking on fish tankmates, but rarely are these recorded as being ultimately deleterious to the host. More likely this behavior is a manifestation of food deprivation, and a sign that you need to be supplementing their diet with more greens.


Due their intolerance of cycling metabolites and fondness for algae these fishes should only be placed in well-established systems; ones where the plants have become firmly settled and are growing.

Otocinclusshould be purchased in a group (one or two per ten gallons of system is about right functionally), being social animals, and batch-processed via a dip and quarantine procedure (Fenner, 1989,94).

Predator/Prey Relations

Most all small to medium size and aggressive fishes get along with Otos. Barbs, Gouramis, loaches and other bottom feeders. Larger cichlids and generally agonistic species might well be tempted to try sucking them up; for their tanks, try the larger Loricariids.

Reproduction, Sexual Differentiation:

Sexing Otos is not difficult, but takes a trained eye. Due to their small size, the definitive gill cover spines and pointed genital papilla of males is hard to make out. Instead look to the females; they are decidedly broader and rounder in appearance.

Due to mixed species shipments, the wannabe breeder is encouraged to purchase and stock Otocinclus in as large a grouping as practical, and provide propitious circumstances (food, habitat, water quality) for nature to take its course.

Otos are listed by some authors as "egg scatterers" contrasting with other egg-laying Loricariid genera. I side with Speice, 1987, Emmens and Axelrod, 1978, Sands, 1988 list them as breeders of adhesive eggs on rocks, glass aquarium surfaces, and broad leaved plants respectively. Baensch and Riehl in their Atlases state that pairs do a "dance", embrace and about 50 transparent to cream to yellowish green eggs are laid in an upper corner of their tank. Young hatch out in 2-4 days... they list baby brine and spinach as first foods.

Feeding/Foods/Nutrition: Types, Frequency, Amount, Wastes

Inadequate feeding/nutrition is only second to selecting decent specimens as a source of mortality with these fishes. Three key points; do not place Otocinclus in new, or overly cleaned systems, they will perish from lack of food. Secondly, you must expressly provide foodstuffs for these little suckers, (e.g. sinking pellets, greens like spinach, Nori, peas) especially if keeping them in an unplanted or under-planted setting. And lastly, the "wood trick"; that is, supplying your Otos with some submerged wood as a needed dietary adjunct.

Disease: Infectious, Parasitic, Nutritional, Genetic, Social

As far as being members of the South American sucker mouth catfishes go, Otocinclus are barely armored against poor water quality. This shows up in their ready susceptibility to "organic pollution". For a dearth of water changes, otherwise adequate filtration, Otos die from bacterial-population-explosion "disease". Maintain high water quality.

On import, Internal worms, mainly of the gastrointestinal tract and gill flukes can be lethal. Don't trust your livestock source to have done the job on these. During quarantine, feed your new Otocinclus anti-parasitic food for a good week.

The too-common infestations of ich can be quickly resolved with malachite with or without formaldehyde preparations. Be wary of utilizing salt, metal (copper) or insecticide (DTHP, Masoten, Dylox, Neguvon) medications on Otocinclus; they succumb to these treatments more readily than the apparent cause you're treating.


The miniature Suckermouth catfishes of the genus Otocinclus vie with the cyprinid SAE's (Siamese algae eaters) of the genus Crossocheilus for the title "world's most useful algae eaters" for planted aquariums. They are peaceful, unobtrusive, and industrious little suckers; and readily available and inexpensive.

If you intend to put them to good use, just keep in mind the four cardinal points offered here. Pick out healthy stock, keep water quality high and constant, provide a suitable habitat, and suitable food including some submerged wood.

Bibliography/Further Reading:

Baensch, Hans A. & Rudiger Riehl. 1993. Aquarium Atlas, v.2. BAENSCH, Germany. 1212pp.

Burgess, Warren E. 1989. An Atlas of Freshwater and Marine Catfishes: A Preliminary Survey of the Siluriformes. T.F.H. Publications, Inc. NJ. 784pp.

Castro, Alfred D. 1996. Algae al fresco: Exactly which algae eater really likes to chomp down on this stuff? AFM 12/96.

Davis, Chuck. 1979. Choosing suitable catfish. FAMA 6/79.

Emmens, C.W. & Herbert R. Axelrod. 1978. Catfishes, 3d ed. T.F.H. Publications, Inc., NJ. 96pp.

Evers, H.G. 1996. Der hummel-harnischwels O. gibbosus Ribiero, 1908- Ein ausser gewohnlicher wels. TI Magazin 128 Apr. 96, 24-26.

Fenner, Bob & Steve Landino. 1989. Acclimating fishes. FAMA 8/89.

Fenner, Bob. 1994. Guerrilla acclimation methods, or acclimating: My way. FAMA 10/94.

Feuer, Warren. 1995. Rating the Loricariids: You have algae, but will they eat it? Aquarium Fish Magazine 10/95.

Ferraris, Carl. 1988. Basic catfish nutrition: the myth that catfish can live quite nicely on food ignored by other tankmates is simply not true. AFM 10/88.

Finley, Lee. 1993. Catfish corner: Some notes on feeding Suckermouth catfishes. TFH 8/93.

Finley, Lee. 1997. Catfish corner: An overview of catfishes bred in the aquarium-part two. TFH 4/97.

Frank, Neil & Liisa Sarakontu. 1995. Algae eating cyprinids from Thailand and neighboring areas (Crossocheilus). The Aquatic Gardener 8(2):3,4/95 and AFM 4/96.

Gosline, William A. 1947. Contributions to the classification of the Loricariid catfishes. Archos. Mus. Nat., Rio de Janeiro, 41:79-144.

Isbrucker, I.J.H. 1980. Classification and catalogue of the mailed Loricariidae. Verslagen en Technicsche Gegevens. No. 22. 181 pp.

Kutty, Vinny. 1993. Algae eaters. The Aquatic Gardener 6(3):5,6/93.

Nelson, Joseph S. 1994. Fishes of the World, 3d ed.. John Wiley & Sons, Inc. NY. 600pp.

Padovani, Gian. 1988. The Catfish. R/C Modeler Corp., CA. 78pp.

Regan, C. Tate. 1904. A monograph of the fishes of the family Loricariidae. Trans. Zool. Soc. London, Vol. 17:191-351.

Riehl, Rudiger & Hans A. Baensch. 1982 (sixth ed. 1996). Aquarium Atlas, v.1. MERGUS, Germany. 992pp.

Riehl, Rudiger & Hans A. Baensch. 1996. Aquarium Atlas, v.3. MERGUS, Germany. 1104pp.

Sands, David D. 1984. Loricariidae: Notes from the score of nature. TFH 10/84.

Sands, David. 1988. A Fishkeeper's Guide to South American Catfishes. Tetra Press, NJ. 117pp.

Schmidt, S. 1996. Prime breeder O. paulinus- an eater of algae with strange tastes. Aquarium (Hilversum) 66(4):100-102.

Speice, Paul. 1987. Guppies to groupers: There's a sucker born every minute (Loricariidae). FAMA 2/87.

Thomas, Scott B. 1978. Suckermouth catfishes: Nature's algae removers. FAMA 12/78.

Wheeler, Stu. 1978. When your catfish calls in sick. FAMA 3/78.

Otocinclus with Veracity Dear Sir, I just wanted to say thank you for your article concerning the Otocinclus at http://www.wetwebmedia.com/FWSubWebIndex/otocinclusart.htm. Not only is it well written and easily understandable, but you also took the time to write your references, which gives the article credibility (a rare thing on the internet). I am certain my Otocinclus will benefit greatly from the time you have given to write this text. Thanks again :) Guillaume Pomerleau :) <Thank you for your kind, encouraging words. Am hoping to continue writing such in the years to come. Bob Fenner>  

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