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The Oscar Fish: Astronotus ocellatus [/TD]
by Brett Harrington (aka Fogelhund)

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Organism Fish Fin Adaptation Marine biology

Interestingly enough, the Oscar was described as Lobotes ocellatus in 1829-1831 by Baron de Cuvier. The original description appears in the publication by Spix and Agassiz, Selecta
Piscium Brasiliam
129 Pl. 6. Lobotes is in fact a salt-water genus, and it was described that the Oscar as occurring "at Ocean, off Brazil". In the early 1800's, it wasn't uncommon that fish were collected, and taken to ichthyologists, sometimes hundreds of miles away. They relied on being told where the fish were collected, and sometimes the information wasn't exactly accurate.

The scientific description evolved as follows; Lobotes ocellatus Cuvier, 1831, Cychla rubro-ocellata Jardine, 1843, Acara compressus Cope, 1872, Acara hyposticta Cope, 1878, Astronotus ocellatus var. zebra Pellegrin, 1904, Astronotus orbiculatus Haseman, 1911 and finally to Astronotus ocellatus.

Astronotus means being marked with a star on the back, and ocellatus bearing an "eye spot" marking. Plenty has been written on the possible function of these markings, but no acceptable conclusions have been drawn, and it hasn't been demonstrated that these markings have any function at all.

Now that we've taken care of all that scientific nomenclature, let's talk about the fish themselves. Oscars have been a popular aquarium fish for many years, and it's no wonder given their seemingly intelligent behaviour, and ability to respond to people. Under healthy circumstances, an Oscar will easily grow above 12", with 14-15" being very common for healthy adults. Very large specimens have been known to grow to 18".

The natural range for Oscars is South America in the Amazon basin of Peru, Columbia and Brazil, but is also known from the French Guinea. In the wild they have a wide range of water parameters, from ph of 6.0-8.0 and a dh of 5.0 - 19.0. Temperatures are typically around 22-25 C. Most Oscars do best in a ph of 7.5 or less, conditions they have been bred for generations in.

If you give your Oscar clean water conditions, room to grow and thrive, and feed it properly, there is no reason that it shouldn't live for at least 10 years, and they've been known to last 15 years in some cases.

The natural diet of Oscars & feeding your Oscars in the aquarium

The Oscar is a carnivorous fish in the wild. It is not built to be a piscivore, with its slow and methodical movement and predictably up to 80 percent of its diet consists of invertebrates. In nature, they primarily feed on crustaceans, gastropods, aquatic insects, and insect larvae, but will also eat juvenile fish when they get a chance.

When feeding your Oscar at home, the most important factor in choosing what to feed, should be the long-term health of your pet. It is a commonly known fact that Oscars can, and will just about eat anything. But just as you and I should not dine at McDonald's every night, care should be taken to ensure that the Oscar receives a beneficial diet. A diet that mimics the nutrition a wild fish would eat is a good start.

Earthworms, mealworms, soft-shelled crickets, moths, flies, bloodworms, snails, ghost shrimp and crayfish would all make a great meal for an Oscar. Mysis shrimp, Krill, and Human consumption shrimp are also wonderful ideas. Many people will also feed feeder fish, such as goldfish and guppies to their Oscars. Commercial pellets and flakes are also readily available, with good nutrition, as part of an Oscar's diet. It is better to feed an Oscar 8 -12 medium sized pellets, then 6 large ones.

Photograph Organism Fin Fish Underwater

I have mentioned that the long-term health of your pet should be the primary factor in deciding which of the available foods you should choose. Many of the choices are live foods, which can be purchased, or collected on your own. Take care in ensuring that the food you are feeding is free of disease that is common with feeder fish, such as ich & parasites, which will be passed on to your Oscar. Studies show that there is little nutritional value in feeder fish. Given the risk of infecting your pet with a disease, one should think very hard about whether feeder fish should be part of your Oscars diet. We would recommend against it. If you are collecting bugs, you will also wish to be cognisant of any Pesticides, or chemicals that they could have been exposed to, which could potentially harm, or even be lethal if fed to your fish.

There are many options available for feeding your Oscar, though it is recommended to provide a good assortment of different foods for your fish. Variety is the spice of life, and this is no different for an Oscar.

The Non-native Oscar

In the late 1950's, there was a deliberate attempt to stock Oscar's in Dade County, by Fish Farmers. It is now quite established in Southern Florida, throughout a number of counties. It is in direct competition with a number of natural species, such as sunfish, and has the capacity to cause irreversible harm on the native ecosystem. The Northern range of these feral Oscar's appears to be limited by temperature. If water temperatures drop below 12.9 C, it is believed to be lethal to Oscars.

It is never a proud moment in human history when we cause harm to a native ecosystem, and this is true whether these be by deliberate means, or accidental means. If you have an unwanted pet, whether it is an Oscar or other fish or animal, please act responsibly to ensure the safety of the environment that you live in. These type releases can cause severe environmental problems and as a result introduction of any exotic fish is illegal in Florida and punishable by a second degree misdemeanor punishable by up to a $500 fine and/or 60 days in jail.

Diseases of the Oscar

Oscars can and do suffer from similar ailments of other fish, including ich, velvet, or fungus. This is not meant to be an exhaustive resource on Oscars, and these basic diseases are not normally lethal if correctly treated. If your Oscar comes down with one of these "minor" ailments, contact your favorite retailer or the Cichlid-Forum Discussion Board to discuss treatment.

What is a common, and potential lethal disease of Oscars is "Hole-in-the-Head" (HITH), also referred to as HLLE these days, "Head and Lateral Line Erosion". We'll stick with just HLLE from here on. The disease affects the areas around the lateral lines, and sensory organs in the face, causing small "holes" or pits in both areas. The disease remains a bit of a mystery though, and studies conducted in this area have mostly been inconclusive and/or contradictory. We shall deal with some of the theories that have been at least partially substantiated. First, let's talk about some theories we believe don't hold any water.


The theory that Carbon can contribute to, or lead to HLLE is questioned in the scientific community. Tests to validate this theory have not consistently had results that can prove carbon is a factor in HLLE. Yet, there is some anecdotal evidence exists where aquarists have introduced carbon, with the result being HLLE in their fish, and upon removal had it disappear.
Some people believe that it might be improper cleaning of the carbon that causes the problem with carbon, as the "carbon dust". Still others believe that the ongoing use of carbon can lead to the removal of vital elements in the water, with these deficiencies leading to the illness.


Many old texts refer to the flagellate Hexamita as the blame for HLLE. Hexamita is a cold water parasite not found in cichlids and therefore cannot be the cause. A similar flagellate found in cichlids - Spironucleus vortens, has been found in the intestines of cichlids. Spironucleus has also been fingered as the blame for HLLE.

I've had several discussions with Lee Newman, Curator of Tropical Fishes of the Vancouver Aquarium regarding HLLE. Lee has noted that the Aquarium has had a number of specimens tested over the years who have had HLLE, and that none of the tests from the lab showed any evidence of Spironucleus within these fish. So, we can safely conclude that Spironucleus is not the cause of HLLE.

So what causes HLLE?

After conducting a great deal of research, I've come to the conclusion that the cause of HLLE is in fact stress, and I had the full agreement of Lee Newman who had similarly come to this conclusion.


Stress for an Oscar can be broken down into three basic categories. Food, Water parameters, and stress from inappropriate tank mates.


The diet of fish is the most accepted cause of HLLE that finds support in the scientific community. It is important to ensure that you're Oscar, or any fish, is receiving a balanced and proper diet. Studies have shown links to the development of HLLE with a lack of Vitamins C&D, as well as lacking in phosphorous and calcium. Most commercially prepared pellets and foods are rich in these nutrients, so a varied diet again is recommended to prevent HLLE. As previously noted, Feeder fish are lacking in nutritional value, and are also known to contain enzymes that break down Thiamine, an important vitamin to your Oscars.


Water quality is of paramount importance in an Oscar aquarium. While it hasn't been scientifically proven, poor water quality is likely the main contributor to poor health and HLLE in Oscars. Maintaining good water quality in an aquarium with fish as large as an Oscar requires plenty of mechanical and biological filtration. Good water quality comes as a result of having a healthy and active population of beneficial bacteria, which can be maintained through proper biological filtration and water changes. More frequent and smaller water changes are usually better then less frequent and larger water changes. Oscars have been known to be sensitive to chorines and chloramines, as well as some heavy metals. If your water source, whether it be city water, or well water is high in any of these, you may wish to purchase a water filter, or chemicals, to reduce any harm that may come to your fish from these chemicals. Having some water movement and surface water movement will contribute to the water quality and oxygenation of the aquarium.

The ideal water parameters for an Oscar is a ph certainly less then 8.0, and preferably 7.5 or lower. Ammonia and Nitrites should be reading at 0, with nitrates less then 40 ppm, and less then 10 being optimal. There have been studies which have shown that where water conditions are not kept within these parameters, HLLE can develop, even in the absence of other possible explanations. Proper filtration, with water movements and regular water changes cannot be stressed enough.

Tankmate selection

The third area of stress is that of inappropriate tank mates. Situations where the tank is overstocked, or where an Oscar is perpetually bullied will cause stress on your fish. Purchasing the correct size aquarium and stocking it for from the beginning for full adult sized fish is a strong recommendation. Also avoiding fish that are larger or stronger then an Oscar would be recommended. Not only would the constant bullying be a cause, but so to would being regularly out competed for food.

Treating HLLE

  1. Add 1 tablespoon of aquarium salt for every 5 gallons of water.

    The use of kosher salt, marine/aquarium salt of other untreated forms of salt are ok, but table salt is not. If you do any water changes, be aware of adding in any salt that will have been lost in the water change.
  2. Raise the water temperature to over 80 Fahrenheit, but no more then 82 Fahrenheit.
  3. Do a 30% water change, every 3 days.
  4. Feed a balanced and varied diet, rich with vitamins (or a quality prepared food). If necessary, add in vitamin supplements.

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