Otocinclus / oto (Otocinclus vittatus) care guide

Quick facts

Adult size: 4 cm
Lifespan: can live up to 5 years; captive average 3-5 years if they survive the first month
Tank zone: bottom
Temperament: peaceful
Difficulty: intermediate
Schooling: recommended 6+ (critical minimum 4, thrives at 8+)
Typically wild-caught: yes - acclimate slowly

Water parameters

Temperature: 22–26°C
pH: 6.0 to 7.5
Hardness: 2 to 15 dGH

Tank requirements

Minimum volume: 60 L
Minimum length: 60 cm
Flow: moderate
Lighting: moderate
Substrate: any
Driftwood: preferred
Hiding spots: needed

Feeding

Diet: herbivore, feeds primarily at the all.

Biofilm and soft algae are the natural diet. Otos graze constantly and need a mature tank with established surfaces. A brand-new tank cannot support otos. Supplement with blanched zucchini, cucumber, and algae wafers. Repashy Soilent Green is excellent because it sticks to surfaces and mimics the biofilm-grazing behavior. Otos starve in clean tanks that lack algae and biofilm; a fat belly is the sign of a well-fed oto. A concave belly means it's starving and needs intervention immediately.

Vegetable matter required (algae wafers, blanched zucchini, spinach).

Compatibility

Peaceful with everything. One of the few fish safe with even the smallest shrimp fry.
Works well in planted tanks, nano tanks, and community setups. Will not bother any other fish.
Avoid housing with aggressive or very active fish that will stress them. Otos are timid and need calm tankmates.
Sometimes attaches to flat-bodied fish (discus) and rasps their slime coat. This is rare and usually indicates severe hunger.
Keep in groups of 6+ of the same species. Solitary otos are stressed and hide constantly.

Habitat

Multiple species sold under the name "oto" or "otocinclus," most commonly Otocinclus vittatus and O. macrospilus. Native to small, clear streams in South America (Brazil, Peru, Colombia, Argentina) where they graze algae and biofilm on rocks and submerged wood. Almost all otos in the trade are wild-caught, and the capture and shipping process is harsh. Mortality in the first two weeks after purchase is high (estimated 50-60% in some shipments), which has earned them a reputation as fragile. Survivors are actually quite hardy once established. They're the smallest effective algae-eating fish, reaching only 3–4 cm. Groups of 6+ are more confident and more visible.

Breeding

Rarely bred in captivity, though it has been done. The details are poorly documented because it usually happens by accident in mature, heavily planted tanks. Males are reportedly smaller and slimmer than females. Eggs are deposited on glass or plant leaves and are very small. Fry need abundant biofilm. The difficulty of breeding is a welfare concern because wild collection is the source for virtually all fish in the trade, and shipping mortality is significant.

Common problems

Starvation is the primary killer. Otos that die within a month of purchase were usually already starving or weakened from the supply chain. Choose otos with rounded bellies at the store; avoid any with concave bellies or clamped fins. Acclimate slowly (drip acclimation over 2+ hours). Do not add otos to a tank that's been running less than 3 months. Internal parasites from wild collection cause wasting in some fish; there's no reliable prophylactic treatment. Sensitivity to poor water quality (ammonia, nitrite, high nitrate) is high. Otos are often the first fish to die in a neglected tank.

Bioload

Bioload coefficient: 0.8 (4 cm grazer with light feeding; mostly subsists on biofilm so produces less waste than active swimmers of similar size).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.