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Encyclopedia / Algae Eaters / Macrospilus oto
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Variations

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Macrospilus oto

Otocinclus macrospilus · Macrospilus, Otocinclus macrospilus, Dwarf oto, Oto

Peaceful grazer; likes a group of 6 or more.

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3.5 cmBody length, without tail ✓
21–26 °CTemperature ✓
6–7.5pH ✓
40 LSmallest tank ✓
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Tank mates
Check your own mix →
Not recommended
  • Tinfoil barb eats animals up to about 5 cm. Macrospilus oto only grows to 3.5 cm.
Not recommended
  • Giant gourami eats animals up to about 10 cm. Macrospilus oto only grows to 3.5 cm.
Not recommended
  • Angelfish eats animals up to about 4 cm. Macrospilus oto only grows to 3.5 cm.
With conditions
  • Only a narrow temperature range suits both: 21–22 °C.
With conditions
  • Only a narrow pH range suits both: exactly 7.5.
Compatible
Compatible
Compatible
Compatible
Compatible

350 species in our list are a good match.

Facts checked against sources on 16 Oct 2026. ✓ means a source backs it; ? means no reliable source yet. See sources

Care and water

Hardness ✓1–12 dGH
Group size ✓6 or more
Temperament ✓Peaceful
Swims at ✓All levels
Care level ✓Moderate
Diet Herbivore
Lifespan ?Unknown
Size in shops 2–3 cm
Growth Medium

Behavior

Peaceful grazer; likes a group of 6 or more.

Food

Algae and biofilm on glass and wood; blanched courgette, spirulina wafers and algae-based pellets.

Tap a food to see what it is.

Male or female

Best sexed from above: mature females look broader in the body.

Health

Common problems. Tap one to see the signs. This is a guide, not a diagnosis.

Breeding

Egg-scatterer · Hard

Rarely bred; almost all otos on sale are wild-caught.

Pairing

Keep a group with a mix of sexes and feed lots of algae to bring them into condition.

Setup

40 L tank with fine sand, dense plants, some tannins and a gentle current.

What starts it

Cool, fresh water changes and plenty of biofilm and algae feeding.

Eggs

After a Corydoras-style T-position spawning, eggs are scattered around the tank, usually among plants in the upper areas or on the glass. Large cool water changes may trigger spawning; the fry are tiny and need plenty of algae.

Raising the fry

Fry feed on biofilm and powdered spirulina; very prone to starvation, so keep a mature algae-rich tank.

Common mistakes
  • Feeding too little algae
  • Raising fry in a sterile tank
Full breeding guide, day-by-day plan and simulator →

Where it lives in the wild

RegionSouth America – upper Amazon (Peru, Ecuador, Colombia & Brazil)
Rivers and lakesUpper Amazon tributaries
HabitatShallow streams with plants
Native rangeAbout 500 km across
See it on the world map

The story of the macrospilus oto

One of the commonest otos in the hobby, from the upper Amazon, recognised by a large round black blotch at the base of the tail. Imports are delicate (often starved in transit) and do best in mature, planted tanks with plenty of algae and biofilm.

Carl Eigenmann and William Ray Allen described this oto in 1942 from the upper Amazon. Its large tail spot gives it the name macrospilus, meaning big spot. It is one of the otos most often imported for aquarium algae control.

First described by Carl H. Eigenmann & William R. Allen in 1942. Named for 'macrospilus' means large spot.

  • Its tail spot is larger than in most other Otocinclus.
  • It grazes soft algae and biofilm from leaves and wood.
  • It is a social fish that does best in groups.

Sources

16 facts checked with sources