Otocinclus catfish (commonly called “otos”) are the smallest and most peaceful algae eaters available for freshwater aquariums. At only 1.5–2 inches, they fit in tanks as small as 10 gallons, they ignore plants entirely, and they coexist peacefully with everything from shrimp to bettas. For planted tank keepers who want algae control without the bulk of a pleco, otos are the first choice.
But otos have a reputation for being fragile — especially in the first two weeks after purchase. Understanding why they struggle initially, and how to keep them thriving long-term, makes the difference between a thriving group and repeated losses.
Species Overview
- Scientific name: Otocinclus vittatus (most common), O. cocama, O. macrospilus (less common)
- Maximum size: 1.5–2 inches
- Lifespan: 3–5 years
- Temperament: Peaceful, shoaling
- Minimum group: 6+ (they are social and stressed alone)
- Minimum tank: 10 gallons for a group of 6
Tank Requirements
Water Parameters
- Temperature: 72–79°F (prefer 74–77°F)
- pH: 6.0–7.5
- GH: 3–15 dGH
- KH: 2–8 dKH
- Ammonia/Nitrite: 0 ppm (absolutely critical — otos are very sensitive)
- Nitrate: Under 20 ppm
Tank Setup
Otos thrive in well-established planted tanks. They need:
- Mature tank with biofilm. Otos graze biofilm constantly. A new tank with no biofilm cannot support them. Wait at least 2–3 months after cycling before adding otos.
- Broad-leaf plants. Anubias, java fern, and large crypts provide grazing surfaces where algae and biofilm develop. More leaf surface area = more food.
- Driftwood. Otos graze the biofilm that develops on driftwood surfaces. Aged driftwood covered in a thin film is an oto buffet.
- Smooth rocks. River stones develop algae and biofilm that otos consume.
- Gentle flow. Otos come from well-oxygenated streams and appreciate some water movement, but not strong current that prevents them from holding position on surfaces.
What to Avoid
- New or sterile tanks. No biofilm = no food = starving otos.
- Bare tanks. Otos need surfaces to graze. A bare-bottom tank with no plants or hardscape is unsuitable.
- Strong chemicals. Avoid medications containing copper in oto tanks. They are sensitive.
- Aggressive tank mates. Otos are extremely timid and will stop feeding if bullied.
Diet and Feeding
Primary Diet: Biofilm and Soft Algae
In a well-planted, mature tank, otos graze biofilm and soft algae continuously throughout the day. This is their primary and preferred food source. The thin, invisible film of bacteria, diatoms, and microorganisms that coats every surface in an established tank is what otos evolved to eat.
Soft green algae and diatoms (brown algae) on glass, leaves, and hardscape are eagerly consumed. Otos are not effective against hard algae types like black beard algae or green spot algae — those are too tough for their small mouths.
Supplemental Feeding
Most tanks cannot sustain a group of 6+ otos on biofilm alone long-term. Once they eat through the available algae and biofilm, they need supplemental food:
- Blanched vegetables: Zucchini slices, cucumber rounds, spinach leaves. Blanch 30 seconds to soften. Attach to a veggie clip or weigh down with a rock. Remove uneaten portions after 24 hours.
- Repashy Soilent Green or Community: Gel foods that coat surfaces and mimic biofilm. This is the single best supplemental food for otos.
- Algae wafers: Hikari Algae Wafers or similar, broken into small pieces. Some otos take to these readily; others ignore them initially.
- Spirulina flakes: Crushed and dropped onto smooth surfaces where otos feed.
Feeding Tips
- Otos are shy eaters. They often wait until lights are off to visit supplemental food. Offer food at lights-out.
- Rotate food types. Otos that refuse algae wafers may eat Repashy, and vice versa.
- Watch bellies. A healthy oto has a slightly rounded belly. A flat or concave belly indicates starvation — increase feeding immediately.
- Competition matters. In a tank with corydoras or plecos that eat supplemental foods faster, otos may not get enough. Offer food in multiple locations.
The First Two Weeks Problem
Otos have high mortality in the first 1–2 weeks after purchase. This is primarily due to:
- Starvation during shipping. Otos need to graze constantly. During the shipping process (which can take days), they have no food. Many arrive already weakened.
- Wild-caught stress. Most otos in the hobby are wild-caught, not captive bred. The capture, holding, and shipping process is enormously stressful.
- Depleted fat reserves. Otos carry very little body fat. A few days without food puts them in critical condition faster than other fish species.
- Chemical damage. Otos are sometimes collected using chemicals that weaken them.
How to Maximize Survival
- Buy from a store that has had them in stock for 2+ weeks. The weakest fish die during holding. Fish that have survived 2 weeks at a store are likely to survive in your tank.
- Quarantine with food. During quarantine, provide multiple food sources (Repashy on the glass, blanched zucchini, algae-covered rocks from your main tank). The priority is getting them eating immediately.
- Add to a mature tank only. Never add otos to a tank less than 2 months old. They need established biofilm.
- Acclimate slowly. Drip acclimate for 1–2 hours. Otos are very sensitive to parameter differences.
- Buy in groups. A group of 6+ feels more secure and feeds more confidently than 1–2 alone.
Tank Mates
Otos are compatible with virtually all peaceful community fish and invertebrates:
Ideal Tank Mates
- Neocaridina and Caridina shrimp — zero competition or aggression
- Small tetras (ember, neon, cardinal, rummy nose)
- Rasboras (harlequin, chili, exclamation point)
- Corydoras catfish — similar peaceful bottom-dweller temperament
- Bettas (usually peaceful with otos, though monitor initially)
- Guppies, endlers, and small livebearers
- Snails (nerites, mystery snails)
Avoid
- Large cichlids — may eat or harass otos
- Aggressive fish — anything that chases or nips
- Large plecos — may out-compete otos for food and territory
Breeding Otocinclus
Breeding otos in captivity is rare but possible. It happens most often in well-established, heavily planted tanks with excellent water quality.
Conditions That Trigger Spawning
- Large water change (50%+) with slightly cooler water — simulates rain
- Soft, slightly acidic water (pH 6.0–6.5, GH 3–6)
- Well-conditioned adults with varied diet
- Dense plant cover (especially java moss and fine-leaved plants)
- Group of 6+ with multiple males
Spawning Behavior
Males chase females around the tank, nudging their sides. The female deposits individual eggs (tiny, about 1mm) on glass, leaves, or flat surfaces. Clutch size is small — typically 30–50 eggs per spawning event.
Raising Fry
Eggs hatch in 3–5 days at 78°F. Fry are extremely small and feed on biofilm and infusoria. A well-established tank with plenty of aged surfaces gives fry the best survival odds. Supplemental feeding with powdered spirulina or Repashy Super Green can help.
Fry survival in community tanks is possible if the tank is densely planted and adults are well-fed (so they do not eat eggs or fry). Dedicated breeding tanks improve success significantly.
Frequently Asked Questions
How many otocinclus should I keep together?
Minimum 6, ideally 8–10. Otos are social fish that feel more secure and feed more actively in larger groups. A lone oto often hides, stops eating, and declines. The bigger the group, the more natural and active their behavior.
Do otocinclus eat plants?
No. Otos eat algae and biofilm on plant surfaces but never damage the plant tissue itself. They are one of the most plant-safe fish in the hobby. Even delicate mosses and thin-leaved plants are safe with otos.
Why is my oto’s belly concave?
Starvation. A flat or concave belly means the oto is not getting enough food. Immediately provide supplemental food — Repashy Soilent Green on the glass, blanched vegetables, and algae wafers. Also check that other fish are not out-competing the otos for food.
Can I keep otos in a 5-gallon tank?
A 10-gallon tank is the minimum for a group of 6. A 5-gallon does not provide enough surface area for biofilm production or swimming space for a proper group. In a 5-gallon, otos will deplete available food too quickly.
Do otos eat black beard algae?
No. Otos eat soft green algae, diatoms (brown algae), and biofilm. They cannot handle tough algae species like black beard algae, staghorn, or green spot algae. For BBA control, look at Siamese algae eaters or manual removal.
Are otocinclus shrimp safe?
Completely. Otos have no interest in shrimp of any size, including baby shrimplets. They are among the most shrimp-compatible fish available and are routinely kept in Neocaridina and Caridina breeding colonies.
Conclusion
Otocinclus are outstanding algae eaters for planted tanks when their specific needs are met: a mature tank with established biofilm, a group of 6+, supplemental feeding, and peaceful tank mates. The key to success is buying healthy specimens from a store that has held them for at least two weeks, adding them to an established (not new) tank, and ensuring food is always available.
Once past the risky first two weeks, otos are surprisingly long-lived and rewarding fish. A group of 8 otos busily grazing every surface in a planted tank is a sight that never gets old.