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Otto Fish - Description and requirements in aquariums

Brief Description

The “Otto Catfish” is one of the most popular additions to many planted tanks but unfortunately one of the most misunderstood fish that keepers decide to purchase. They never seem to live for long periods and keepers say that this fish is too delicate to be kept but often it is the keeper themselves that are not providing the correct conditions for this wonderful little fish and if housed correctly they can be kept for their full lifespan and be a superb member of your community planted tank.


Hopefully this article will explain how to care for these and explain all that you need to know as regards their welfare and suitable tank mates.

The most commonly species of “Otto” that is kept by keepers is the Otocinclus macrospilus, these are only found in the upper Amazonian River Basins of Peru where they inhabit the heavily vegetated bank areas of the rivers and creeks. They can also be found under floating vegetation that are common in these river basins where they live together in large groups as they are a very social fish. There is another common species which is the Otocinclus cocama, commonly called the Zebra Otocinclus and both species inhabit the same areas and have the same requirements. The Otocinclus macrospilus used to be sold under the name of Otocinclus affinis but this has since been reclassified and the name is no longer used. They belong to the family Loricariidae along with hundreds of other catfish species but these are one of the smallest of the catfish , adults will only reach a maximum size of 1.5 inches (3.81 cm). This is another reason that they are so popular as a group of these can be kept in a smaller aquarium quite happily meaning that they are affordable to a larger range of fish keepers.

They do have one or two unique features that are often missed due to their small size, they possess a pair of nostrils on the top of their nose that they use to smell their food, these are very small but are visible with the naked eye and often the sense of smell allows these fish to find food at night when their tank mates are resting.

On their head there are two minute whiskers that are sensory organs, these are very small and not as developed as with other whiskered catfish but they do have a function as they can detect water movement. Their mouths consist of four plates that are used for rasping away at rocks or the tank glass as algae forms a large part of their diet.

One of their most endearing features are there ventral fins that they use to grasp onto plant leaves and suchlike, they look comical when they are gripping the leaf with their fins wrapped around it. One unusual feature of their anatomy that cannot be seen too clearly with the naked eye is hair like structures that are all over their bodies. These are called “odontodes” and are actually formed from bony material, these can get stuck in fishnets so it is best to use another method when attempting to catch these fish. As of yet nobody fully understands the purpose of these, one school of thought is that they may be there to ward off predators not wanting to run the risk of getting the fish stuck in their throats.

Setting up the aquarium for Otto catfish

As mentioned above, these fish are a social species and as such need to be kept in small groups, a tank as small as 15 gallons (~ 68 litres, 18 US gallons) will allow enough space for these fish to thrive but if you are planning on adding these to a community tank then a larger aquarium will be required. The temperature needs to be set at a range between 21 - 26°C (69.80 - 78.80°F) so in warmer climates there may not even be the need to add a heater, the pH of the water should range from 5.5-7.5 so this is quite a wide scope that the Otocinclus are happy with. They love heavily planted tanks, especially if the plants are broad leaved, add a few rock piles so that they also have the chance to graze on any algae that may be present. One important point to remember is that they need to be added to a mature set up, they will not do well in newly set up tanks as they are very sensitive to any swings in water parameters and any sign of ammonia or nitrites in the water will lead to early mortalities. The nitrates need to be kept below 20 ppm so regular water changes must be performed, always change at least 10% of the water weekly. Tank mates should only include peaceful species as the Otocinclus cannot compete for food or stand up to any sign of aggression. Adding fish such as Corydoras or peaceful tetras is the ideal situation.

Purchasing your Otocinclus

Unfortunately these fish do not travel well and as most specimens sold are wild caught they are often in a bad shape when they reach the aquatic stores. You need to select healthy fish as keeping these fish for over a month in your aquarium is usually the hardest part, after that the Otocinclus should survive for their full lifetime unless the keeper makes a huge mistake with their tank maintenance. Only select specimens that have a full body shape, they often arrive with sunken bellies, these fish need to eat constantly and are often starved while being shipped. Make sure that they have been quarantined properly and are full of colour, there are many diseases that can strike fish and the Otocinclus does not have the best of immune systems even when in full health.

When you have selected healthy specimens you will need to acclimatise them slowly into your tank and try to get them to eat as soon as possible, it is often a good idea to leave the lighting switched on in your aquarium for long periods a few days before adding these fish so that there is algae there waiting for them to fill up their bellies.

Feeding your Otto fish

Against all fish keeping rules Otocinclus need to be kept fat, they should never show signs of starvation, they primary food source will always be algae, nearly always there will never be enough algae in the aquarium to keep these fish well fed, if this is the case then the diet needs to be supplemented with other food sources. The easiest way to keep a constant supply of algae in your aquarium is by adding algae wafers every now and again, the Otto do not normally feed from the substrate but even these fish find it hard to resist a snack of one of these wafers, if there are other bottom dwellers such as Corydoras make sure that there are enough wafers to keep all of the fish fed.

Some flake food is also high in vegetable content, particularly marine flake, there are traces of salt in this food but not enough to do any harm to freshwater fish. Fresh vegetables can also be added to the diet, zucchini,cucumber, blanched spinach and romaine lettuce can all be used but you may find that the Otto fish may ignore these until the have been in the aquarium for a few hours and have softened down a bit making them easier to digest.

Breeding Otocinclus catfish

There are only a few scant reports of these breeding in the aquarium, recreating ideal surroundings for this to occur is very difficult but there are a few keepers who claim to have had some form of success with the actual spawning but I have yet to see any reports of fry actually being raised successfully. The few reports that I have seen claim that these fish breed in a similar fashion to Corydoras which is quite believable, the Otocinclus catfish can only really be sexed by viewing the fish from above, the females will have a rounder body shape compared to the males and are sometimes slightly larger. The females will swim up and down potential spawning sites such as plant leaves, rocks, tank glass and even pipework from the filters with the male closely following behind. It has been observed that these fish, just like Corydoras will assume the “T” position as the male tries to encourage the female to deposit her eggs and this spawning ritual takes place in open water. The eggs are sticky and attach to the chosen site b y means of a thread ready for hatching. There are no detailed reports of how long the eggs take to hatch but it is quite common for a keeper to suddenly notice an extra fish in the tank as a result of the Otto being very secretive when breeding.


Hopefully this article will help you keep these wonderful catfish but the main points to remember are:-

The first month of them being added to the tank are the most crucial, if they survive through this then they will live for considerably longer.

Always keep these fish well supplied with a high vegetable diet and plenty of algae to consume.

Keep the water quality as high as you can and never add these fish to a tank that hasn’t matured.

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