Longfin Oscar

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Oscar fish - Astronotus ocellatus

Scientific name: Astronotus ocellatus

Common name: Oscar fish

Family: Cichlidae

Usual size in fish tanks: 30 - 40 cm (11.81 - 15.75 inch)


Recommended pH range for the species: 6.5 - 6.5

Recommended water hardness (dGH): 4 - 18°N (71.43 - 321.43ppm)

0°C 32°F30°C 86°F

Recommended temperature: 24 - 30 °C (75.2 - 86°F)

The way how these fish reproduce: Spawning

Where the species comes from: South America

Temperament to its own species: peaceful

Temperament toward other fish species: peaceful

Usual place in the tank: Middle levels

Food and feeding

The Oscar will eat anything that will fit in its mouth; they are not very picky eaters. Astronotus ocellatus are omnivorous and they should be mainly fed by insects, crustaceans and worms (use less fish based food) as well as veggie matter and their diet should be high on vitamin C. Great are high quality cichlid pellets or sticks with frozen fish food like krill. Brineshrimps, bloodworms and so on are good for young fish, but they are just too small for the adult ones. Some other stuff you can feed them include prawns, shrimps (but not tiger shrimps), clean earthworms or crickets and from vegetable mattered food try peas, spinach, cucumber, melon, zucchini or even fruit like oranges and bananas. Some specimens will eat them, some not. Keep the diet varied and try more kinds of food to see which your Oscar likes the most. Don’t use any poultry or mammal parts like beef hearts, since their metabolism cannot handle it. Goldfish are not a good choice either, because their meat is high on fat and contains thiaminase which destroys vitamin B. Other feeder fish might pose health risks since those sold in stores are mostly held in bad conditions and you also should consider that Oscars are clumsy and might hurt themselves while chasing these fish. And how much to feed them? It is recommended to feed them few times a day with small amounts and one day in a week avoid feeding altogether. They are capable of eating enormous amounts of food really quickly, but then they barf it out, eat it again and barf it out again.


Oscars are originally from South America. They can be found in many rivers in the basin of the Amazon river therefore in many South America states. Nowadays they even can be found in some warm climate rivers in USA and Singapore. The preferred habitat are slow or still waters under a heavy growth of vegetation with lots of submerged plants.


Even if there are many supposedly reliable methods how to tell a female from a male, they are mostly ineffective. The only sure way is to observe them while breeding. Obviously the female will be the one with the eggs coming out. Sometimes even two females can mate but that doesn’t always mean that they both will lay eggs so unless the eggs are fertilized (they turn yellow), you can’t be sure the other fish is really a male.


The hardest part is to separate a male and a female, since it is so hard to tell a fish’ sex and they have to like each other too, but otherwise it is not that difficult. Place the pair in a suitably large tank with some flat rock on the bottom, subdued lighting and no tank mates - once the eggs are laid removing the tank mates might cause the parents to eat their eggs. To induce spawning, slowly rise the temperature and do large water changes to simulate warm spring rains. When the mating starts, the pair will start swimming around each other, their colour brightens and they might seem more aggressive toward each other, all the while they will clean the breeding spot. 24 hours after the eggs are laid - there can be up to 2 000 of them - the fertilized ones will turn yellow-orange. Until then the eggs will be white and the non-fertilized ones will stay white too. It is very likely that the inexperienced parents will eat the eggs or the fry at first, but as they try again they will get better and they will take care for their young too. But if they don’t, you can take the eggs out into a different tank, but you have to somehow simulate the fanning parents do. Eggs will hatch within 3 days, but for another 4 days the fry will be unable to swim. Sometimes you might think that the eggs went bad and the fry died, but in reality the eggs hatched and the little ones are hiding in the gravel. The fry should be fed by newly hatched brine shrimps and you really have to keep the water very clean. A sponge filter is a safe bet, but you should do daily water changes anyway.


Getting an Oscar is a long term commitment since they can live up to 18 years if you give it the proper care. The main health issue comes from the poor water quality. They are messy and if you underestimate the water, especially the filtration and water changes, you risk many diseases.

Tank requirements

This is a huge fish and it will need a lot of space. About 75 gallons or 300 litres for one fish only and then 50 gallons (200 l) for every additional Oscar. One of the things Oscar keepers love (and hate) is, that they have the mind of their own even when it comes to the tank decorations. They dig holes into the substrate, rearrange decorations or even throw things out of the tank. Considering this, use only live plants if you have something to tie them to and try to choose the hardy ones since Oscars might try to destroy them. Plastic plants should have a weighted base but prepare yourself that they will be tossed around. The best substrate is the sandy one (even if it might clog the filter) or fine gravel. You should choose only big smooth rocks, large driftwood and caves (especially for a young specimen). Use only few decorating pieces and leave the rest of the tank free for swimming and avoid anything with sharp edges since they tend to bump into things. They will also mess with any equipment you put into the tank like internal filter, heater or some submerged light so don’t bother. You will need at least one external filter anyway, one that can filter the whole water volume at least 8 times per hour or more of them with the same effect. Try to find a canister filter with a build-in heater and a large biological medium. Since they like to jump out of the tank, keep a hood above the tank, but protect the light bulbs in case the fish jumps into them. They don’t need bright light, on the contrary, some fish will prefer no light at all. You even might place some toys into the tank for the mental stimulation like ping-pong balls.

Tank mates

Some say that Oscars get lonely in a tank, but that is not true, they will be quite fine alone. If you want more fish, take in mind that as this fish is one of the giant ones, it will see smaller tank mates as snacks. You should also know that in the nature their diet consists mainly of catfish. The best tank mates will be similar sized fish, but try to avoid too aggressive ones since despite of the size of the fish, it is peaceful. However if you put more Oscars into one tank, it can be a gamble as they might not like each other and get aggressive. This is more of a possibility when they grow up because mature ones like solitude more. Avoid having three Oscars in one tank as it almost always ends up by one fish being bullied by the other two ones.