Sunday,26 September 2021, 6 : 18 AM

Astronotus ocellatus

Oscar:Astronotus ocellatus
Image Credit: Hayath M


Astronotus ocellatus is a popular aquarium fish.
It is commonly referred by its common name "Oscar" .


(Click for details of Cichlid taxonomy)

Oscars are member of Genus Astronotus which is placed in subfamily Astronotinae.
This species was originally described by Louis Agassiz in 1831 as Lobotes ocellatus, as he mistakenly believed the species was marine.
Later on species was placed in genus Astronotus.

The species name is derived from latin word ocellatus which means "little eyes" , this is in reference to red ringed circular spots found at the base of caudal(tail) fin.


Astronotus ocellatus is a large fish and has been reported to grow to about 45 cm (18 in) in length and 1.6 kilograms (3.5 lb) in weight.

  • Shape :-

    Astronotus ocellatus has a Standard Length of approximate 14 " and Total Length 0f 16"
    The body shape is moderately elongate and laterally compressed.
    It is widest in region of anterior(front) of flank and posterior part of head.
    When viewed from the front the body outline forms a narrow oval.
    The front part of pre dorsal outline raises sharply, the second part is less steep, outline gradually ascends though edge of dorsal-fin scale sheath to soft dorsal fin.
    The first part of dorsal fin is composed of spines and the base outline slopes from about middle of spinous portion.
    Pre pelvic contour is little convex, it is nearly as steep as pre dorsal contour.
    Abdominal edge is straight, sloping slightly, the fish body is deepest at anal fin origin
    Body depth is 48 % of SL.

  • Head :-

    Head is short and is approximately 28% of TL.

  • Snout :-

    Snout is short and when viewed from side it forms a blunt square, it narrows off at the top (dorsal aspect).

  • Eyes :-

    Eyes are widely placed, Inter orbital space is nearly as wide as mouth.
    Eye placement is slightly below forehead outline, and is entirely in upper and anterior halves of head.
    Inter orbital space (between eyes) is flat.
    Pre dorsal outline rises sharply till middle of eye orbit then less steeply.

  • Mouth :-

    Mouth is large, upper jaw (maxilla) reaches nearly 3/4 of orbit (eyes).
    Tip of upper jaw ( maxilla) is exposed, lower jaw is placed below middle of orbital(eyes).
    Lower jaw protrudes slightly.
    Both lips are interrupted and thick they are referred as "African lips".

  • Scales :-

    Scales are cycloid mostly in pre dorsal region, abdomen scales are ctenoid.
    Lateral line is broken and has 20-23 scales.

  • FINS

    Dorsal Fin:-
    Dorsal fin is combined with spines on first dorsal and soft rays in posterior part.
    Dorsal fin base outline slopes gently.
    First dorsal-fin spine is inserted slightly in advance of vertical from hind margin of operculum.
    The length of spines increases to fourth spine.
    Soft Dorsal fin has a rounded tip, does not reach to the middle of caudal fin.
    Dorsal fin is heavily scaled.
    There are 12 -14 dorsal spines.
    Dorsal soft rays are 19-21.

    Anal Fin :-

    Anal fin is a single unpaired fin, and it originates just opposite soft dorsal fin.
    Soft portion is similar to soft dorsal fine, but it does not reach beyond the middle of caudal fin.
    There are 3 anal spines and 15-17 soft rays.
    Anal fin is also scaled like dorsal fin.

  • Pectoral Fin : -

    Pectoral fins occur in pair.
    Pectoral fin has a blunt dorsal tip.
    4th ray is largest and hind margin is truncate or slightly curved.
    There are 15-16 soft rays.
    Pectoral fins are naked, i.e without any scales however sometimes a small cycloid scale may be present between 6th and 7th ray.

  • Pelvic Fin :-

    Pelvic fin spine is inserted just below pectoral axilla.
    Fin is pointed, outer branch of first ray is longest, reaching to to first anal spine and 1/3 of soft anal fin base.
    Pelvic fin is covered by cycloid scales.

  • Caudal Fin :-

    Hind edge of caudal fin is rounded.
    Caudal fin is completely covered by scales.

  • Gill rakers :-

    Gill rakers are short and thick with many dentils.
    First gill arch is without a lobe.

  • Teeth:-

    Like all cichlids Oscar have jaw teeth as well as pharyngeal teeth.
    Teeth in jaws are small are conical, pointed, little re curved and used to grab food.
    Teeth in first half in each jaw are stronger than the rest.
    There are few but strong teeth in lower pharyngeal tooth-plate.

  • Coloration(wild):-

    Basic body ground colour is light grayish, chest is dirty silver, abdominal midline is whitish.
    Operculum and cheek are grey-brown, snout and forehead are brown.
    Lower head is silvery over layered with grey-brown.
    There is a blackish stripe which continues along pre orbital margin on cheek and it vanishes just before reaching preopercular corner.
    There are minute dark brown dots marginally on gill-cover and adjacent pectoral girdle dorsal to pectoral axilla.
    Sides are adorned with dark (brown or gray brown) vertical bars.
    A wide vertical bar of ground colour runs across side above vent and spinous anal fin.
    The vertical bars on mid flank are usually divided on abdominal sides.
    There is a dark band running to pelvic-fin base, there is a faintly continuous identical bar on opposite side.
    A faint dark band runs across throat between posterior part of interoperculae, just behind light vertical mid flank bar.
    There are 1-2 wide dark bars above anal fin, more or less divided or otherwise of irregular shape, and a dark vertical blotch or bar over end of caudal peduncle.
    Black scale edges are more or less evident, chiefly in dark markings.
    Fins are grayish.
    The basal scaly part of dorsal fin has a series of black silver ringed spots of varying irregular appearance.
    A large black spot surrounded by an orange ring present on either side of the base of the upper caudal peduncle is a striking characteristic of this species.
    These ocelli have been suggested to function to limit fin-nipping by piranha (Serrasalmus spp.), which co-occur with A. ocellatus in its natural environment.
    The external part of pectoral fin has black or, usually, gray large spot over bases of middle rays.
    The internal part of pectoral fin has a much more intense, deep black blotch covering bases of rays except about 5 ventral and a greater or lesser part of axilla.
    The species is also able to rapidly alter its coloration, a trait which facilitates ritualized territorial and combat behaviors amongst con specifics.
    Juvenile oscars have a different coloration from adults, and are striped with white and orange wavy bands and have spotted heads.

  • Distribution & Habitat:-

    Oscar are found in Amazon River basin in Peru, Colombia and Brazil; French Guiana.
    In its natural environment, the species typically occurs in slow-moving white-water habitats, and has been observed sheltering under submerged branches.
    The species is limited in its distribution by its intolerance of cooler water temperatures, the lower lethal limit for the species is 12.9 C (55.2 F).

  • Sexual Dimorphism

    Although the species is widely regarded as sexually monomorphic,males have been suggested to grow more quickly, and in some naturally occurring strains, males are noted to possess dark blotches on the base of the dorsal fin.

Keeping in Aquarium

Tank Size:

The most important aspect of mixing any aggressive cichlid in an aquarium is the tank size.
The minimum size tank that is universally recommended for one full grown oscar is 55 gallons(200 lit) .
In that size tank, no other tank mates can be recommended.
There is not enough territory for two fish to establish separate claims, and therefore the aggression would be excessive.

In a 75(280 lit) gallon tank a few tank mates are acceptable.
One to two small cichlids (between 3-5 inches) or one medium cichlid (between 6-8 inches) is acceptable.
The determining factor, of course, is will the oscar accept them.
Highly aggressive males are not good candidates for tank mates, even in a 75g.
When you mix cichlids there will always be some level of aggression and needs to be carefully watched.

In a 125(480 lit) gallon tank there are many options.
The recommended stock list from various veteran aquarists shows a wide variation.
A Major factor for this can be the fact the Oscar are highly individualistic temperament.
A 125 gallon tank can house 3 large cichlids (~12 inches), 5-6 medium cichlids, and between 10-15 small cichlids; all of this barring any excessive aggression or breeding.


Oscars are very messy feeders and have a very heavy bio load.
Hence the filtration should be very good.
Good quality External canister filters are often used in Oscar aquariums.

Substrate & Decor:

When setting up an oscar tank, a few things need to be kept in mind.
First, Oscars love to redecorate.
If the object in aquarium is small enough for the oscars to move, it probably will be moved at one point or another.
This rules out anything that is remotely breakable, such as ceramic ornamentation.
It also rules out most plants (real or fake), since they will be killed/destroyed over time by the oscars. Live plants will not survive long.
Fake plants can be used, but don't get elaborate in their use because whatever concepts you have in decorating your tank, the Oscar will have his or her own, and anything you do will eventually be undone by the Oscar.

Another consideration is that oscars move around a lot and are fairly clumsy.
This means that they bump into things a lot.
That rules out anything with sharp edges and/or rough textures, such as lava rock.
Oscars have a habit of scratching themselves up or even ripping out large chunks of flesh, either of which, for a fish, can become life threatening.

Finally, remember that Oscars are soft water fish.
Unless your water is extremely soft, you will want to avoid anything that will tend to raise the pH of the water in the tank.
This rules out any calcium carbonate-based minerals such as coral, aragonite, limestone, marble, sea sand, and sea shells.

The ideal items would be:

  • Rounded big rocks, these should be big enough so that oscar cannot move them, and the rounded edges will ensure that the fish does not hurt them.
  • Flat rocks, if you manage to get a a pair ready to breed, some flat rocks are necessary for the pair to clean and lay eggs.
    In absence of flat surfaces Oscars are known to dig in the sand right up to the bottom glass looking for a flat surface.
  • Driftwood is another option.
    The tannins in the driftwood tend to buffer the water to the lower pH range that Oscars prefer.
    It should not have thin protruding edges and the piece should be big enough, preferably it should be anchored to a piece of flat rock like slate.

Water Parameters:

Oscar needs soft water and acidic water with a pH of 6.5-6.9.
The species live in tropical South American rivers and the recommended water temperature in the aquarium is 21-28C / 70-83F.
If you decide to keep the water in the upper part of the range, don't forget that warm water holds less oxygen than cooler water.
If your filtration system isn't enough to keep the oxygen level up you can get one or several air stone.


Oscar are carnivorous by nature and may be fed prepared fish food designed for large carnivorous fish, crayfish, worms, and insects (such as flies or grasshoppers).
Feeding live foods may increase the rate of growth but also may cause endoparasites.
Poultry and/or mammalian flesh, including beef heart, should not be fed long term as these fatty foods will contribute to fatty liver disease.
Since these fish eat fruit in the wild, items such as melons, oranges, and other fruits can also be used as a type of food.
Just about anything that falls into the water would be eaten by oscars.
Live feeder fish can be given, but fish such as goldfish and rosy red feeder minnows should not be fed.
These contain an enzyme (thiaminase) within their flesh which binds vitamin B1, leading to deficiency.

Though oscars have a predominantly carnivorous but they are not active hunters.
Most fish eaten by A. ocellatus in the wild are relatively sedentary catfish, including Bunocephalus, Rineloricaria, and Ochmacanthus species.
The species uses a suction mechanism to capture prey, and has been reported to exhibit "laying-on-side" death mimicry in a similar fashion to Parachromis friedrichsthalii and Nimbochromis livingstonii.
The species also has an absolute requirement for vitamin C, and develops health problems in its absence.

Tank Mates:

Oscars are said to be fish with great personality.
They are often said to provide the same companionship which a dog can provide.
There is flip side to this great personality, you never know if they will like to have another fish in their tank.
It is always a touch and go in keeping other fish with them, it might work or it might not work.

A good strategy is to purchase the fish young and keep it with a companion of similar size, as the two fish grow, there is a better chance of the Oscar accepting its tank mate.

Following are some choices for a tank mate for Oscar:-

  • Multiple oscars:
    Aquarium size=125g

    The bare minimum tank size two oscars is 125 gallons.
    Mixing oscars is really just like rolling dice, it's a gamble.
    Sometimes you'll have two that get along great and sometimes you'll have two that cannot live together peacefully even in a 6ft tank.
    The pairing process can be misleading as well.
    Most juvenile oscars are quite social and will often swim side by side with tank mates as if they were inseparable.
    However, when oscars mature they often become more fond of solitude and can become aggressive toward a former mate.
    Keeping more than two oscars can be tricky.
    The number three is seemingly an unlucky number when it comes to these particular fish.
    The situation almost always plays out as two oscars teaming up and bullying the third oscar relentlessly.
    Naturally, one would think that the two fish that pair up are male and female, but this is not always the case.
    Sometimes two females team up and bully a male; sometimes two males team up and bully a female.

  • Convict Cichlid
    Amatitlania nigrofasciatum
    Aquarium size=75g

    The convict really is the perfect tank mate for an oscar.
    They are small as compared to the large bulky oscar and will not contribute significantly to the bio load of the aquarium.
    The majority of convicts are also very bold, and resilient.
    Not only are they aggressive enough to live with a much larger tank mate, but they are tough enough to brush off the occasional chase.

  • Firemouth Cichlid
    Thorichthys meeki
    Aquarium size=75g

    Firemouths are similar in size to convicts so they compliment the giants size.
    They are not quite as bold or as aggressive as convicts.
    For that reason some will shy away from confrontation and hide.

  • Jack Dempsey Cichlid
    Rocio octofasciatum
    Aquarium size=75g

    Dempseys can be good tank mates for oscars.
    They are a medium sized cichlid.
    Some can be extremely aggressive, and some can be very timid.
    Some males have also been known to reach 12 inches in length though 8-10 is more common).

  • Jaguar Cichlid
    Parachromis managuensis
    Aquarium size=125g

    Jaguar cichlids have a bad reputation for being overly aggressive.
    Many of them are very aggressive, but some are fairly laid back.
    In a 6ft tank, the two species have a decent shot at getting along peacefully.

  • Salvini Cichlid
    Nandopis salvini
    Aquarium size=75g

    Salvinis have been described as a 'reclusive' fish when they are not breeding or defending their territory.
    They can be excessively aggressive, but their cautious nature keeps them hidden most of the time.

  • Severum Cichlid
    Heros efasciatus
    Aquarium size=75g

    Severums are relatively docile fish.
    They will mostly keep to themselves, but won't be pushed around.
    Although they can reach 10-12 inches, they have a very thin profile and shouldn't overwhelm the bio load.

  • Red Devil Cichlid
    Amphilophus labiatus
    Aquarium size=125g

    Red devil are big, bulky, aggressive fish.
    Even in a 6ft tank, its hard to predict success with an oscar.
    Though it can work, mixing these two fish is a risk.

  • Silver Dollar
    Metynnis argenteu

    Silver Dollars are good candidates for dither fish.
    Their unique body shape prevents them from being eaten even at a small size, and their thin body profiles keep their addition to the bioload of the aquarium minimal.
    Silver dollars find comfort in numbers so keeping them in shoals of less than 3 is not recommended.
    They are active swimmers and compliment the slow moving/lurching of the oscar.

  • Giant Danios
    Devario aequipinnatus

    Giant Danios are another decent choice for dither fish.
    They are small, but very fast.
    They are also schooling fish and due to their small size should be kept in a larger shoal.
    A minimum of 6 should be kept.

  • Tinfoil Barb
    Barbonymus schwanenfeldii

    Tin Foil Barbs are a hazardous choice, a large tank is required, a 6ft tank bare minimum.
    Tinfoil barbs can and do exceed 12 inches in length.
    They are also very active fast swimmers that, like the others, prefer to be kept in groups no less than 3.
    These fish grow fast, but when they are young, could easily be eaten by an adult oscar.
    Be wary of this choice in dither fish.

  • Spotted/Four-line catfish
    Pimelodus pictus/blochii

    Pictus catfish Pictus Cats are a dangerous choice.
    They are active swimmers and good scavengers but prefer to be kept in groups.
    Their relatively small size (5 inches) is what makes them a gamble with your oscar.
    Eating a pictus cat might be oscars last mistake.
    Both Spotted and Four-line pictus cats have bony pectoral protrusions that they use for defense.
    Not only are they sharp at the tip of the bone, but they are also jagged on the anterior surface making it extremely difficult to remove should this weapon puncture any tissue.
    A four-line pictus is a safer choice as they grow to 8-9 inches in length, but still their long thin bodies make them a target for determined oscars.

  • Spotted/Striped Raphael catfish
    Agamyxis pectinifrons

    Raphaels are another option as a tank mates.
    Striped raphael cats can grow to 8 inches, and Spotted raphael cats grow to 5 inches roughly.
    Both of these cats have hard armored bodies that deter most predators from eating them, this is true for oscars as well.
    The downside of both Raphael cats is that they tend to be very nocturnal.
    They hide for the duration of the day, and really are only active when the lights are out.

  • Featherfin Syno
    Synodontis eupterus

    They are a peaceful species that reach 8-9 inches in length.
    This is one of the most compatible tank mates for large cichlids as far as the common catfish is concerned.

  • Pleco

    The Common plecostomus has got to be one the most purchased fish in the hobby today.
    These fish are purchased with the mind set that they will "clean your tank".
    However this is a misconception.

    Most plecostomus are more messy than your average fish.
    They do eat algae that grows on the sides of aquarium, but they also create a lot of waste and add a large portion to the bioload of the tank.
    There are two species of plecostomus sold as "Common Plecos": Hypostomus plecostomus, and Pterygoplichthys gibbiceps.
    The H. plecostomus normally max out around 12 inches, while the P. gibbiceps grow to 18-20 inches.
    Either way you look at it, these are large messy fish.
    Even though they may live peacefully with an oscar, I would avoid these fish, especially if you only want one to clean the tank.
    Those who choose the common pleco should be prepared to do heavy, frequent water maintenance.


The species reaches sexual maturity at approximately one year of age, and continues to reproduce for nine to ten years.
Frequency and timing of spawning may be related to the occurrence of rain.
Oscar are bi parental substrate spawners, though detailed information regarding their reproduction in the wild is scarce.

The closely related Astronotus crassipinnis has been observed, in times of danger, to store brood in its mouth, possibly for protection, in a manner reminiscent of mouth brooding Geophaginae cichlids.
This behavior, however, has not yet been observed in A. ocellatus.

In captivity, pairs are known to select and clean generally flattened horizontal or vertical surfaces on which to lay their 1,000 to 3,000 eggs.
Like most cichlids, A. ocellatus practices brood care, although the duration of brood care in the wild remains unknown.

Oscars will lay their eggs on a flat surface in a single layer (no egg will be on top of another one).
A flat rock like slate is ideal, or they have been known to clear the gravel and lay on the bare bottom of the tank.
They will work for days picking up the gravel building mountains in the corners.
Between "digging" they will go thru lip locking, shaking, quivering and sometimes aggression towards each other.

Their color variances also intensify during this period.
However the female may lay eggs regardless of whether there is another fish in the tank or not.
It is also possible to have two females and have one or the other lay eggs.
To further confuse things, two females may become aggressive towards each other while one, or the other, or both are laying eggs.
This sometimes looks like mating behavior between a male and female.

Fertile eggs will be amber in color after 24 hours.
White eggs are infertile.

If the eggs are white, take the slate out or clean the tank.

Oscars are very good parents once they get some practice.
New parents might raise the fry right away but unlikely.
The eggs might be eaten or they might let the eggs hatch to the wiggler stage and then eat them, or if there is any other fish in the tank, the fry might be eaten.
If there are any other fish in tank, take the eggs out immediately.

When there is a confirmed breeding pair, give them a tank to themselves.
Parents are known to eat eggs and even fry if there is danger from other fish in the tank.
When they get a hang of raising fry they teach the fry to come to the front of the tank for feedings and if the fry stray too far away they will suck them in to there mouths and place them in the "pits".

If the parents keep eating the eggs or fry you can take the eggs out and hatch in a 10 gallon bare bottom tank.
Make sure the tank is established and the water temp matches the parents tank.
Use tank water in a bucket to make sure eggs are covered while moving them.

Sponge filter are best for fry tank, there is no danger of fry getting sucked in, also after some time the sponge get covered with a layer of algae and small particles of food which serve as a source of food for fry.

Place the slate upright at the back of the tank and place an air stone in at the base to agitate the water in front of it.
This will mimic the fanning that the parents do.
Within a few days you might see a thickening or a fungus growing on top of the eggs.
This thickens as the eggs hatch.
You can use suitable anti fungal medications meant for this purpose.
After the eggs are laid, you should be able to see wigglers within 2 to 3 days.
Shut the tank lights off and use a flashlight to look around at different angles.
If you don't see wigglers the eggs were not fertile, fungus killed them or they have moved off the slate into there hiding spot.
There might be times that you think the fry are dead or have been eaten but they are in fact hiding in the gravel pits, underneath the gravel or behind the slate.

Brine shrimp is a good fry for freshly hatched fry.
Keeping Clean Water is of super importance.
Do a 25% water change daily cleaning the bottom of the tank to remove all food residue.
The best way is to use an airline to suck it out into a cup, bowl or pitcher.
Then siphon the babies that you pick up back into the tank.
It is almost impossible to clean the tank bottom without sucking up a few of the wigglers.
When you refill the tank, be sure that the water you put in is the same temp or maybe just a little warmer than the tank. Siphon it in with an airline slowly to minimize the effects of any variances in water temperature.

Species Snapshot

Species Card

Particulars Details
Scientific Name Astronotus ocellatus
Common Name Oscar
Genus Astronotus
Subfamily Astronotinae
Geographical Origin Rio Ucayali drainage & upper Amazon river of Peru and Brazil
Diet Carnivore
Gender Differences Mono morphhic
Breeding Substrate Spawner
Temperament Aggressive
Con Specific Temperament Aggressive
Water hardness Soft, ph range: 6-7, dH range: 5-19
Difficulty Level in Aquariums 3

Morphometric Data

Particulars Details
Total Length(inches) 17.00
Standard Length(inches) 14.00

Measurements as % of Total Length

Particulars Details
Body Depth 39.67 %
Head length 27.89 %
Pre Dorsal length 29.40 %
Pre Pectoral length 28.20 %
Pre Anal length 64.60 %
Pre Pelvic length 31.80 %
Caudal Peduncle Depth 14.77 %
Caudal Peduncle Length 6.23 %
Fork length 100.00 %
Length of Last Dorsal Fin Spine 10.96 %
Pectoral Fin Length 27.97 %
Pelvic Fin Length 29.96 %

Measurements as % of Head Length

Particulars Details
Head Width 17.18 %
Snout length 5.81 %
Eye diameter 26.80 %
Pre Orbital Length 22.30 %
Inter Orbital Width 12.37 %
Pre Orbital Depth 22.30 %

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