Siamese Algae Eater vs. Otocinclus

© Swapan Photography/

Written by Kyle Glatz

Published: October 31, 2022

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Pet fish enthusiasts often introduce fish into their aquariums because they are fascinating to watch. Sometimes, though, people keep pet fish for a specific use.

Algae-eating fish are helpful because they reduce the number of algae in the aquarium, reducing the amount of commercial food required for the fish while increasing the diversity of their tanks.

In this article, we will compare the Siamese algae eater vs. Otocinclus and show how these two types of algae-eating fish are unique! 

Comparing a Siamese Algae Eater and Otocinclus

Siamese Algae EaterOtocinclus
SizeLength: up to 6 inchesLength: about 1 inch to 2.1 inches
Morphology– One dorsal fin and two ventral fins
– Forked tail fin
– Grayish brown color with a slightly flattened underside
– Dark stripe running the length of the fish that fades when they’re stressed
– Has two thin barbels near the mouth
– One dorsal fin, two ventral fins
– Some species have adaptations that allow them to breathe air
– Looks vary between species
– May have a mottled coloration, but most have a black stripe down their sides
– Flattened head like most other catfish
– Forked tail
Diet– Algae, phytoplankton, and more
– Kept as pets for their algae diet, especially because they’re the only fish to eat red algae
– They will possibly stop eating algae if they are offered too many other foods
– Algae, algae wafers, and some other plant matter like vegetables  
SpeciesCrossocheilus genus
Crossocheilus oblongus
– Member of the carp family
Otocinclus genus
– Member of the catfish family
– 19 species of Otocinclus exist
Natural Distribution– Primarily live in Southeast Asia
– Found in Malaysia, China, Thailand, and other countries- Found in lowland fresh water in South America
– Particularly found east of the Andes Mountains

The Key Differences Between a Siamese Algae Eater vs. Otocinclus

The Siamese algae eater is more prominent than all species of 


, growing up to 6 inches long.

©Grigorev Mikhail/

The most significant differences between a Siamese algae eater and Otocinclus are their size, eating preferences, and morphology. The Siamese algae eater is more prominent than all species of Otocinclus, growing up to 6 inches long compared to the high-end 2.1 inches of Otocinclus.

The Otocinclus, being a member of the catfish family, has a flattened head, mottled colors on its body, and a sucker mouth. Some species can even breathe oxygen. Meanwhile, Siamese algae eaters are known for their brownish-gray bodies and the dark stripe that runs their body length, which fades when they’re stressed. They have two small barbels by their mouths as well. Their eating behaviors are also unique. The Siamese algae eater can eat red algae, but the Otocinclus does not. Meanwhile, the Otocinclus prefers to eat algae and other plant matter, including vegetables.

These differences can help you tell these creatures apart by behaviors and looks. 

Siamese Algae Eater vs. Otocinclus: Size

The average Siamese algae eater is larger than the Otocinclus. The Siamese algae eater measures up to 6 inches in length, but the many species of Otocinclus measure between 1 inch and 2.1 inches long. That means the Siamese algae eater is much larger than the members of Otocinclus.

Siamese Algae Eater vs. Otocinclus: Morphology

The Otocinclus and Siamese algae eater have some things in common with their bodies, like sharing the number of dorsal and ventral fins and having forked tails. However, the Otocinclus has a flat head like other members of the catfish family, a trait not shared with the Siamese algae eater.

The Otocinclus has a mottled coloration along with a black stripe that goes down its sides, something they have in common with the Siamese algae eater. Yet, the line on the Siamese algae eater can fade when the fish is under duress.

These differences, combined with the size of the fish, can be used to differentiate these particular creatures.

Siamese Algae Eater vs. Otocinclus: Diet

The members of Otocinclus are known for eating algae.



 is located in the fresh lowland waters in South America, from Argentina all the way north to Venezuela.

©Pavaphon Supanantananont/

The members of Otocinclus and the Siamese algae eater are both known for eating algae. However, the Siamese algae eater is known for eating red algae, and members of its genus are the only fish to do so. They make popular pets as a result of their diet. However, they also eat plankton and algae wafers. Yet, if they are fed other foods aside from algae, they can lose their taste for it.

Meanwhile, the Otocinclus genus is known for eating algae, algae wafers, and other plant matter, including vegetables like zucchini. They will maintain their love of algae and won’t deviate from it. 

Siamese Algae Eater vs. Otocinclus: Species

The Siamese algae eater is a member of the Crossocheilus genus, and its official name is Crossocheilus oblongus. This fish is a member of the carp family. However, the Otocinclus genus, a genus of the catfish family, has 19 different species. These two fish are very different in terms of their families and genera.

Siamese Algae Eater vs. Otocinclus: Natural Distribution

The Siamese algae eater naturally lives in Southeast Asia. They are found in the waters in parts of Malaysia, China, Thailand, and a few other countries. Meanwhile, the Otocinclus is located in the fresh lowland waters in South America, from Argentina all the way north to Venezuela.

These fish are not all that rare and are sold to fish enthusiasts throughout much of the world. Some species are hard to breed, so they are often captured and sent to their new owners.

Both the Siamese algae eater and the members of Otocinclus are fascinating fish. They’re great pets when kept with other mellow fish and serve the essential purpose of eating algae to keep the tank from getting too polluted. Yet, these fish are unique from one another in looks and size.

Still, suppose you’re considering getting one of these fish. In that case, evaluating your needs and the type of fish you already have is essential. Otherwise, you could send your new fish into a hostile environment.

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About the Author

Kyle Glatz is a writer at A-Z-Animals where his primary focus is on geography and mammals. Kyle has been writing for researching and writing about animals and numerous other topics for 10 years, and he holds a Bachelor's Degree in English and Education from Rowan University. A resident of New Jersey, Kyle enjoys reading, writing, and playing video games.

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