Otocinclus: The Good, The Bad and Algae

So what exactly is an Oto? It is a tiny green / brown catfish of the genus Otocinclus, that enjoys to munch on algae all day.

Eating algae in itself would be enough recommendation for many tank owners but these fish are also fun to watch, stay small, are affordable and very cute.

Otocinclus care guide


If you have a planted tank, matured, but with the almost inevitable algal breakouts, I’m hoping to sell you on the idea of Otocinclus catfish as the leaders of your natural algal-defence squad. If you have a busy, unplanted, boisterous community tank with large inhabitants I’m going to suggest you look elsewhere.

Otos do need a little special attention and for that reason you probably wouldn’t want to pick it for your first tank. They need really clean water or they just won’t do well and so you need some practice at water quality management.

On the other hand, they are quite unique in that they are really the only algae eating fish available that will stay small-tank sized and stick with an algae diet into adulthood.

Oto pros

  • Hard working algae eaters.
  • Affordable.
  • ​Peaceful and stay small.
  • Characterful.

Oto cons

  • Delicate on arrival, need a special effort to get them settled into your tank.
  • ​Need really clean water. No ammonia, no nitrite and nitrates as low as you can go.
  • ​You need to home a shoal of at least 6 to make them feel at home.
  • ​Vulnerable to big or aggressive fish.
  • ​Have been accused of feeding on slime of Discus and Angels.

If you don’t think Otos are for your tank, think of alternatives such as the Siamese Algae Eater (check on their final size and their compatibility with your other fish) or Ancistrus/Bristlenose which can stand more varied water conditions and more disruption than Otos. Chinese Algae Eaters are worth avoiding because of their highly territorial behaviour.

Oto ID

There are 20 species of Otocinclus, but they share many things in common. They are small sucker-mouthed catfish. Even as adults the largest Otos come in at around only 2 inches long.

They have muted green/brown/grey coloration often with a darker horizontal stripe down their flanks. Their shape is streamlined and some say they resemble a little shark.

Otocinclus Vittatus

Otocinclus Vittatus (Source)

In the pet store trade the different species are often not recognised or correctly identified. Read up from the references I have listed below if you want to make sure what you’re getting. The key to telling some of the more similar varieties apart is the detail of tail markings.

Here’s a list of the most commonly offered Oto species.

  • Otocinclus vittatus – Common Oto, Dwarf sucker, Oto, Oto Cat
  • Otocinclus macrospilus – Common Oto, Dwarf sucker, Oto, Oto Cat
  • Otocinclus cocama – Zebra Oto
  • Otocinclus affinis – Golden Oto
  • Otocinclus mariae – Dwarf Oto
Otocinclus Cocama

Otocinclus Cocama (Source)

To be honest Golden Otos are not very golden, and Dwarf Otos are not very much smaller than other Otos! The one species that is really a bit special is the Zebra Otocinclus which sports smart, vertical stripes. Expect to pay a premium for these guys.

Wild Otos

In the wild Otos swim in big shoals, often of more than a thousand fish, mingling with Corydoras. They spend their time, cruising the fast flowing shallows of streams and small rivers. They are South American, from Colombia in the north to Argentina in the south.

They spend their time grazing on aufwuchs. Aufwuchs is the German name for the slimy coating which grows on the leaves of aquatic plants. Aufwuchs is composed mainly of diatom algae but also includes animals such as rotifers, mini crustaceans and protozoa.

In their natural habitat, Oto Cats are reported to be found feeding from the surface of finely leaved aquatic plants or submerged grasses but also roots and other submerged wood.

They like to keep to shallow water because the high light levels there promote more algae and in all probability there are fewer large predatory fish to bother them.

In-Tank Character

Otocinclus Macrospilus

Otocinclus Macrospilus (Source)

Many Oto owners speak of them affectionately. They seem to have more personality than a lot of small fish. If you keep enough to allow them to do a little shoaling, you will have a lot of interesting hours watching them interact.

They are not nocturnal; I suspect they can be pushed this way in a crowded or boisterous tank but in a peaceful environment you will see a lot of them in daylight hours. The split their time between resting on leaves and bogwood and active feeding.

Otocinclus are a peaceful group of species, but there are enough reports of them attaching themselves to deep bodied fish that this needs to be taken seriously. Discus and Angelfish are the main targets and there is no definite cause, some speculate that the Otos are hungry and attempting to feed from the target’s slime.

In most cases the Oto comes out worst, when the larger fish retaliates. It’s bad for both animals and is easily avoided by not housing Otos with a slow, deep-bodied fish.

Good Tank Mates

  • Corydoras sp.
  • ​Small Rasboras
  • ​Small Tetras
  • ​Amano or Red Cherry Shrimp
Harlequin Rasbora

Harlequin Rasbora (Source)

Bad Tank Mates

  • Any fish that is territorial or aggressive
  • ​Angelfish
  • Discus
Symphysodon Discus

Symphysodon Discus (Source)

Water Quality

For Otos, cleanliness is more important that an exact temperature or pH level. If your tank and filter are properly cycled and you are keeping stocking levels and feeding sensible then your only real additional issue is keeping nitrates low.

The healthily growing plants that you will need to create an Oto-friendly environment will help with this but your main tool will be the water change. Many planted-tank keepers favour 30% or greater water changes a week and this would suit the Otos well too.

  • pH - 6.0-7.5
  • Temperature – 70 to 78° F
  • Fully cycled tank. No ammonia and no nitrite.
  • Very low nitrate levels are best <20ppm.

Care for all the Otocinclus species is similar. Check Planet Catfish for detailed suggestions on a particular species.

Introducing Otos to your Tank

Reading around the web and aquatic magazines you will quickly hear that there is an issue with Otos not settling in after they are brought home from the fish store. Losses are higher than you might expect.

Perhaps, because they are small and timid and need clean water, they just get worn down badly by the catching, shipping and pet store experience. Almost all Otocinclus you will be offered are wild-caught.

Whatever the cause you need to be extra careful with your acclimation and early days’ routine.

  • Acclimate slowly. Invest in an acclimation kit or make your own with airline.
  • Provide really excellent food but in sensible quantities to boost your cats.
  • ​Don’t clean up all your algae before they arrive. Let them help.
  • Make sure you water is pristine and add extra water changes to your routine.

Tank, Equipment and Décor

In all honesty, I don’t think many people set up species tanks for Otos, they normally get bought for their algae eating function, which is a shame as they have many other fine qualities. But, let’s assume Otos were our only residents, talk about how that tank would be set up and it will give you an idea of whether they would fit into your system.

Otocinclus on Cryptocoryne

Otocinclus on Cryptocoryne (Source)

Tank Size

Remember, you really need at least 6 of these guys to make a happy shoal so a nano tank is not ideal. I would suggest, as a minimum, a 10-gallon system. A good sized tank makes it easier to keep water conditions stable and it will give you leeway to find some other tank mates.


Water quality is vital so an excellent filter is essential. An appropriately sized canister filter is a good choice as you can pack in with biological media and it addition it will provide the strong water flow these cats enjoy. You can add to the flow with a small powerhead if necessary.

Tank Hardscape

It is important to protect the fishes’ delicate suckermouths by avoiding sharp surfaces. A substrate of rounded gravel or soft sand is best. Bogwood is a favourite grazing site and should be included. Stone in the tank may be limited as your likely to want to go with a lot of planted areas but vertical pieces of slate that can attract algal growth are a useful addition.


Really go to town on your planting. The Otos won’t eat your plants. They want to graze algae from them so I would suggest some slow growing plant species. Anubias, Java Fern or Echinodorus would be a good starting point. Cabomba is an easy fine-leaved species to grow and would help mimic Otos natural feeding sites.


Algae!!! Yes, you’ve got it. In an ideal world Otocinclus want the exotic sounding aufwuchs they would get in their native rivers. A diet heavy on plant material and light on animal protein. However, even in a large planted tank with a good algal growth you will putting your cats at risk if you don’t offer some additional food.

You can easily provide some extra fresh plant material by offering cucumber, spinach or zucchini. Wash vegetables well to remove any fertilizer or pesticides.

Catfish pellets or algae wafers can be offered to round out the diet. One breeder brought his Otos into condition by feeding brine shrimp in addition to their normal food.


Otos have been bred in tanks. They breed in a similar fashion to Corydoras, the female rushes around pursued by her male and lays groups of 3-6 eggs on plant leaves, bogwood or even the aquarium glass.

Breeding Otocinclus in aquaria is still in its early days and it would be a great thing to achieve in your tank. Raising the fry is challenging because of the need for tiny live foods and algae. There seems to be no particular stimulus for spawning, although some breeders report that it is seasonal.

Last Word

So there are good reasons and bad reasons to buy an Oto. But really, that is no different to most species. I hope I haven’t put you off these cats by highlighting the possible problems. They are lovely characters and in many gentle community tanks they will fit in. Go the extra mile for them though and get the water clean and the environment right to give you the best shot of success.

Images Credits: 1.

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Jak says

Great read, thanks

Joe says

This is awesome information.

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