Species · Catfish & bottom-dwellers
Otocinclus
Otocinclus spp.
The tiny, peaceful algae-eating catfish — wonderful, but only for a mature, algae-rich tank. Sensitive and often wild-caught, so it's a poor choice for a new setup.
Difficulty: IntermediateTropical community tank✓ Fact-checked care guide
Quick facts
| Adult size | ~3.5 cm / 1.4 in |
|---|---|
| Minimum tank | ~40 L / 10 US gal (established, algae-grown) — SeriouslyFish notes a 45 × 30 cm base |
| Temperature | 23–26 °C / 73–79 °F (within the species range of 21–26 °C / 70–79 °F) |
| pH | 5.5–7.5 (soft, acidic to neutral) |
| Hardness | 1–12 dGH (soft water) |
| Filtration & flow | Cycled, gentle filtration; a mature tank with established biofilm is essential |
| Diet | Grazer — biofilm and soft green algae, plus supplementary vegetables and algae wafers |
| Social | Social — keep a group of at least 6 |
| Temperament | Very peaceful and shy |
| Lifespan | 3–5 years |
Overview
The otocinclus ("oto") is a tiny, endearing, genuinely peaceful algae-eater — one of the few fish safe to trust with delicate plants. In a mature planted tank a group of them is a delight.
But it is the opposite of a beginner's first fish: it is sensitive, frequently wild-caught (and stressed on import), and it needs an established tank already growing biofilm and soft algae. Adding otos to a new tank is a common, avoidable loss.
Water & tank
Otos want soft, clean, stable water (pH 5.5–7.5, 1–12 dGH) at 23–26 °C / 73–79 °F — and, above all, a MATURE tank. They graze the biofilm and soft green algae that only an established aquarium grows, so add them months in, never at setup.
A group is happy in ~40 L / 10 US gal (SeriouslyFish notes a 45 × 30 cm base), planted, with gentle flow.
Feeding
They graze biofilm and soft algae, but a tank rarely sustains a group — supplement with blanched vegetables (courgette, cucumber) and algae wafers so they don't starve once the visible algae is gone.
Behaviour & tankmates
Utterly peaceful and safe with plants, shrimp and small community fish. Keep a group of at least 6 — they are shy and do far better in numbers.
Choose plump, active specimens (many arrive thin and stressed) and quarantine them.
What to expect
A settled group grazes busily over glass, leaves and wood. A sunken, hollow belly is the warning sign — otos that run out of grazing quietly starve, so keep vegetables coming.
Myths, kindly busted
“Otos are a good cleanup crew for a new tank.”
The opposite — they need a mature tank with established biofilm/algae, and sensitive wild-caught stock often dies in a new setup.
“They'll live off tank algae forever.”
A group quickly clears the available algae; without supplementary vegetables/wafers they starve.
When to act
We triage, not diagnose — but these are worth a closer look (starting with a water test):
- A sunken or hollow belly — the classic starving-oto sign; add vegetables and wafers immediately.
- New otos hiding, pale and thin — often import stress; keep the tank mature, calm and well-fed, and quarantine arrivals.
Sources & further reading
- SeriouslyFish — Otocinclus macrospilus — Adult ~35 mm SL; 45 × 30 cm base; temperature 21–26 °C / 70–79 °F; pH 5.5–7.5; hardness 1–12 dGH (soft); peaceful shoaling grazer; needs a mature, algae-rich tank.
Written & fact-checked by The Finfern Team against the sources above — kind, general guidance, not a diagnosis. If something looks wrong, test your water and, if it persists, contact a fish-savvy vet.
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