Otocinclus

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: plant safe.

Typically wild-caught; acclimate slowly.

Feeding

Biofilm and soft algae make up the natural diet, and otos spend most of the day grazing whatever surface they are on. They need a tank that has been running long enough for biofilm to cover the hardscape and leaves; a brand-new setup will not feed them. Useful supplements are blanched zucchini or cucumber, sinking algae wafers, and gel foods such as Repashy Soilent Green, which smear onto surfaces and let the fish graze the way they would in the wild. The simplest health check is body shape: a well-fed oto looks rounded behind the gills, while a sunken outline there means the fish is losing weight and needs immediate attention.

Compatibility

• Peaceful and shy. Safe with almost everything, including the smallest shrimp and shrimp fry, because they have no interest in anything bigger than algae
• Suited to planted, nano, and quiet community tanks. They will not interfere with other fish
• Keep them away from boisterous, fast, or aggressive species; constant activity from larger tankmates stresses them and they end up hiding instead of eating
• Occasionally rasp the sides of broad, slow-bodied fish such as discus. This is uncommon and usually means the oto is hungry enough to look anywhere for food
• Strongly social. A group of six or more of the same species behaves much more naturally than a single fish, which tends to hide and stop grazing

Origin and habitat

The genus is spread widely across South America east of the Andes, from northern Venezuela down to northern Argentina, typically near vegetation along the margins of small or medium streams. There are around 19 described species, with O. vittatus, O. macrospilus, and O. vestitus accounting for most of the fish in the trade; O. cocama, the zebra oto, is a strikingly striped species that sells at a premium when it shows up. O. vittatus has the broadest distribution, covering the Amazon, Orinoco, Parana-Paraguay, and Tocantins basins. Fish imported under the name 'O. affinis' are almost always something else, since the true species (now sometimes placed in Macrotocinclus) is restricted to southeastern Brazil and effectively does not reach the trade. Virtually every oto sold is wild-caught, and the journey from collection point to dealer leaves a lot of them weakened or starving by the time they reach a home tank. Adults are about 4 to 5 cm long, which makes them one of the smallest suckermouths in common availability, and unlike most of their armored-catfish relatives they feed during the day. Their diet is narrow by suckermouth standards: soft green algae and brown diatoms suit them, but tougher growths like black beard or green spot algae are mostly ignored. They graze leaf surfaces without damaging the leaves, so they are safe in planted tanks.

Breeding

Almost never bred deliberately in the hobby. Most reports of spawning come from people noticing tiny fry in mature, heavily planted tanks rather than from any structured project. The few details available describe small eggs laid on glass or leaves, with the male reportedly slimmer than the female. Fry need a steady supply of biofilm to graze on. Because there is no reliable captive supply, every oto sold has paid a wild-collection and shipping cost, which is a real welfare issue for the species.

Common problems

By far the most common way to lose otos is starvation rather than disease, and it usually traces back to the shop. Fish that die in the first few weeks were typically already underweight when bought. Look for a rounded belly and active grazing before buying; a sunken, pinched outline behind the gills or constantly clamped fins are warning signs. Acclimate slowly when adding them, give the tank a couple of months to build up biofilm and gentle algae growth before introducing any, and skip them entirely on a brand-new setup. Wild-caught fish often carry internal parasites that cause gradual wasting even though the fish appears to be eating, and there is no good standard prophylactic. Water quality matters: otos are quick to suffer when ammonia, nitrite, or nitrate climbs.

Bioload

Further reading

• Low-tech planted tank from scratch
• The schooling number matters more than you think
• Keeping shrimp with fish: what actually works