The Otocinclus, or oto, is a small suckermouth catfish from South America and one of the better genuine algae-grazers for the planted community tank.
It works over leaves and glass, removing soft algae and biofilm too delicate for larger plecos.
Its intermediate rating is well earned: most otos die not because they are hard to keep but because they are sold half-starved, often wild-caught, and rushed into immature tanks before there is enough biofilm to sustain them.
Housing
Keep a shoal in a well-established, fully cycled tank of at least 75 litres (20 gallons) with a stable, mature growth of algae.
Maintain 22-27C (72-81F), pH 6.0-7.5, and clean, well-oxygenated water with low nitrate.
Never add otos to a brand-new tank; they need an aged setup with soft green algae already growing.
Provide plants, driftwood, and gentle flow.
They are highly sensitive to ammonia and nitrite, so the cycle must be solid before they arrive.
Diet
Otocinclus are dedicated herbivores that graze soft green algae and biofilm.
In tanks with insufficient algae they will starve, so always supplement with blanched vegetables such as courgette, cucumber, and spinach, plus quality algae wafers.
Many new otos arrive emaciated and need immediate, reliable feeding to recover; a sunken belly is a danger sign.
Keep a steady supply of grazing food and vegetables.
They cannot subsist on commercial flake intended for other fish.
Health
The biggest killer of otos is starvation, both before purchase and in algae-poor tanks.
Combined with their sensitivity to poor water and shipping stress, this makes the first few weeks critical.
Choose round-bellied, active individuals and acclimatise them slowly.
They tolerate medications, salt, and copper poorly, so treat with care.
Check belly shape daily: flat or sunken means underfeeding.
In a mature, stable, algae-rich tank with clean water they settle well and live several years.
Temperament
Otocinclus are peaceful, gentle shoaling fish that feel secure and graze openly only in groups of six or more.
Singly or in small numbers they hide and stress, so a true shoal is essential for natural behaviour.
They are harmless to tankmates and well suited to nano and planted communities alongside small tetras, rasboras, shrimp, and corydoras.
Keep them away from boisterous or large fish, which intimidate these small grazers.
A good fit for
Mature, algae-rich planted tanks
Nano and peaceful community setups
Keepers wanting safe algae control on plants
Aquarists who select round-bellied stock
Common mistakes to avoid
Adding to a new tank with no algae/biofilm
Buying thin, starved, hollow-bellied stock
Keeping fewer than 6 (they shoal)
Salt/copper medications they tolerate poorly
Is your enclosure big enough?
Minimum volume75 litres (20 US gallons)
CompanyCannot be kept alone. The minimum is 6, and the space above is for that group.
In a fish tank the patient is usually the water. Test before you treat — dosing a healthy tank for a disease it does not have makes things worse.
Go now
Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.
Now
Fish gasping at the surface, or crowding the filter outflowIncrease surface agitation immediately, then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems.
Now
Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths point at the water or the equipment, not at a pathogen.
Now
Today
Flashing — repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.
Today
White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.
Today
Soon
Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.
A few days
Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.
This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it — and the time to find one is before you need one.
What it really costs
Setup, monthly, first year and across a whole life, itemised and with the electricity worked out from the actual wattage. Published figures for the same animal disagree by a factor of four, so the calculator shows ranges rather than picking one.
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