If you asked us to name the single best algae-eater for a UK community tank, the otocinclus would win — as long as you can keep them alive past the first month. Small (4cm), peaceful with everything including shrimp, genuinely eats brown diatoms and soft green algae, and safe with plants.

The catch: shop-bought otos die at concerning rates in the first 2–4 weeks. Not because they’re delicate as a species — they’re actually reasonably hardy — but because of how they’re supplied. This guide covers proper care AND the buying strategy that dramatically improves survival rates.

Quick facts

• Scientific name: Otocinclus vestitus, O. affinis, O. macrospilus — several species sold as “otos”
• Common names: Otocinclus, oto, dwarf sucker, dwarf sucking catfish
• Adult size: 3–5 cm
• Lifespan: 3–5 years in captivity (many captive fish die in the first month)
• Water temperature: 22–28°C
• pH: 6.0–7.5
• Water hardness: soft to medium (2–15 dGH)
• Minimum tank size: 60L for a group of 4–6
• Social: shoaling; requires groups of at least 4, ideally 6–10

Why they die (the buying problem)

Most otocinclus in UK trade are wild-caught from South American rivers. The journey from wild capture to your local shop typically involves:

• 1–3 weeks in Peruvian/Colombian holding facilities with minimal food
• International flight in cramped bags
• Days to weeks in wholesaler tanks
• More days in shop tanks

By the time they reach you, most are starving, stressed, and dehydrated. Many are already dying. This is why the “first-month otocinclus mortality rate” in UK community tanks is often 40-70%.

Once past that first month, otos in a well-established tank live 3–5 years without issue.

Buying strategy for higher survival:

1. Ask when the shop received them. Buy stock that’s been in the shop for at least 2 weeks — the weakest will have died already, survivors are stable.
2. Only buy visibly plump specimens. Skinny, hollow-bellied otos are already starving. Round bellies indicate they’ve eaten recently and are stable.
3. Look for active fish. Otos should be swimming, grazing, or clinging to glass — not sitting still on the substrate.
4. Buy from independent aquatic shops — they hold stock longer than big-box chains and know when it arrived.
5. Ask for captive-bred (rare but improving) — Tropica and some UK breeders now produce captive-bred otos. Much higher survival rates. £6–£10 each vs £4 for wild-caught.

Tank requirements

Tank size

• Absolute minimum: 60L for a group of 4
• Ideal: 90L+ for a group of 6–10
• They shoal loosely — need a group, not pairs

Tank maturity (critical)

Otos eat biofilm and soft algae — food that only develops in mature tanks. Adding otos to a tank that’s less than 6 weeks old almost guarantees they starve.

Don’t add otos until:

• Tank has been cycled and stable for 6+ weeks
• Visible algae has developed on glass, plants, and hardscape
• Substrate has visible biofilm

If your tank is spotlessly clean and algae-free, otocinclus will die within weeks even with supplemental feeding.

Filtration and flow

• Moderate flow. Otos evolved in flowing rivers, so decent water movement is welcome.
• Fluval U-series internal or Fluval 07-series canister both work.
• Cover intake with a sponge pre-filter — small otos can get sucked into unprotected intakes.

Water chemistry

Cycle the tank properly first — see our UK cycling guide.

• Ammonia / Nitrite: 0 (always) — otos are very sensitive
• Nitrate: under 20 ppm — sensitive to nitrate long-term
• pH: 6.5–7.5, stable
• Temperature: 24–26°C
• Hardness: soft to medium (4–12 dGH)

UK tap water in most areas is suitable without adjustment. Very hard-water areas can add Indian almond leaves for gentle acidification.

Test weekly with an API Freshwater Master Test Kit. Otos are among the first fish to show symptoms when water quality drops.

Substrate

Fine gravel or aquarium sand. Otos rest on the substrate frequently and sharp gravel can damage their bellies.

Plants — essential

Otos need biofilm surfaces. Plants provide it.

• Broad-leaf plants — Amazon sword, Anubias — otos love clinging to broad leaves
• Java fern — attaches to wood, extra surface area
• Bogwood or driftwood — otos graze on wood biofilm

Feeding

Otocinclus are the archetypal algae grazers — but they eat SOFT green and brown algae, biofilm, and diatoms. They do NOT eat black beard algae, staghorn, or hair algae in useful quantities.

In a mature tank, they mostly feed themselves. But supplement:

• Hikari Algae Wafers — 1 wafer per 4–6 otos, 2× per week
• Blanched vegetables — courgette, spinach, kale. Weigh down, leave overnight, remove in the morning
• Repashy Soilent Green — gelled algae food, premium option. Highly recommended.
• Bacter AE — biofilm booster, weekly pinch. Feeds otos indirectly.

Do not rely on algae alone unless you have a large mature tank with visible algae growth. Supplement always.

Tank mates

Otos are peaceful with everything and safe with almost every other community fish and invertebrate.

Excellent tank mates:

• All small tetras (neon, cardinal, ember, harlequin)
• Rasboras
• Corydoras catfish
• Cherry shrimp — otos ignore adult shrimp and shrimplets
• Ghost shrimp
• Nerite snails
• Livebearers (guppies, platies, mollies)
• Kuhli loaches
• Bristlenose plecs
• Bettas (usually — some individual bettas are fine, others attack anything)
• Small gouramis (dwarf gouramis, honey gouramis)

Avoid:

• Any cichlid larger than rams
• Angelfish — will eat adult otos as adults
• Larger aggressive barbs
• Anything with a mouth wider than an oto

Otos are prey animals. Every fish that could eat them, will.

Sexing

Very difficult. Females slightly larger and rounder-bellied than males, especially when full of eggs. Otherwise identical. Buy 6+ and you’ll have both sexes without trying.

Breeding

Otos rarely breed in home aquaria. When they do, it happens in mature planted tanks with soft slightly acidic water — often triggered by a large cool water change (mimicking rainy season).

The rare successful breeding pattern:

• Established planted tank, at least 6 months old
• Group of 6+ otos including confirmed females
• pH 6.5, temperature dropped to 22°C after a 40% water change with cool water
• Females deposit clusters of 5–10 eggs on plant leaves or glass
• Eggs hatch in 3–5 days
• Fry are microscopic; require infusoria for the first weeks

Most UK aquarists never see oto breeding. It’s a nice bonus if it happens, not something to plan for.

Common problems

Sudden death within days of purchase — the “shop mortality” issue. Not preventable through your care; the fish arrived starving. Higher first-month losses are expected.

Emaciated / hollow belly — starving. Feed more, add Repashy Soilent Green, check the tank has visible algae/biofilm. If persistent, may already be too late.

Sitting still on substrate for days — usually severely stressed or starving. Poor prognosis.

Ich (white spot) — otos are scaleless and require half-dose treatment. See our ich guide — use eSHa EXIT at reduced dose.

Rapid respiration, gasping — poor water quality. Test, big water change, dechlorinate.

Comparison to other algae eaters

Species	Eats algae type	Tank size	Notes
Otocinclus	Soft green, diatoms	60L+	Best for community; fragile in first month
Bristlenose plec	Soft green, biofilm	100L+	Bulletproof; larger; leaves diatoms alone
Nerite snails	Diatoms, green spot	Any	Won’t breed in freshwater; ideal complement
Amano shrimp	Hair algae specifically	Shrimp-safe tanks	Best for hair algae
Siamese algae eater	Black beard algae	90L+	Only species that eats BBA reliably

For a typical UK community tank: bristlenose + nerite snails + 4–6 otos = the ideal algae cleanup crew. See our algae guide for the full treatment.

Bottom line

Otocinclus are worth the frustration. The first-month mortality is unavoidable given the supply chain, but survivors thrive for years in the right tank.

Buy 6–8 from a shop that’s held them for at least 2 weeks. Look for round bellies and active swimming. Only add to a mature tank (6+ weeks cycled) with visible algae. Supplement with Repashy and algae wafers regularly. Expect to lose a couple in the first month; the rest live 3–5 years happily.

They’re not glamorous. But every community tank looks better with a small group of otos grazing quietly along the glass.

Frequently asked questions

Why do otocinclus keep dying?

Most are wild-caught and arrive starving and stressed, causing high first-month losses. Buy plump, active specimens held in the shop for two weeks, and only add them to a mature, algae-rich tank.

How many otocinclus should I keep?

At least four to six. Otocinclus are shoaling fish that are stressed when kept alone or in pairs.

Do otocinclus eat all types of algae?

No. They eat soft green algae, brown diatoms and biofilm, but not black beard, hair or staghorn algae. Supplement with algae wafers and Repashy in cleaner tanks.

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Related: get rid of algae in fish tank, best aquarium fish for UK beginners, cherry shrimp care (otos + shrimp = perfect community).

— Jaqline Smalling, FishForever