Oscars: everyone's favourite cichlids

The genus Astronotus contains two species, Astronotus ocellatus and Astronotus crassipinnis. The former is the popular Oscar, a fish that has been bred commercially for decades and is easily obtained in a variety of colour morphs, some natural, others artificial. The second species is only seen when exported from its natural habitat and is only rarely seen in aquarium shops. In most regards the two species are very similar, certainly in terms of maintenance and behaviour. The chief differences between the two species are size and colouration. The common Oscar is a large fish, sometimes exceeding 40 cm in the wild though typically reaching lengths of around 30 cm in aquaria. Astronotus crassipinnis is much smaller, at most about 20 cm in length. In terms of colour, the common Oscar is incredibly variable, but a green-brown base colour highlighted with lighter brown to orange irregular banding tends to be typical. By contrast, Astronotus crassipinnis is predominantly dark grey to black in colour with more or less vertical light grey to white bands. Both fishes sport a prominent eyespot on the base of the tail.

Ecology in the wild

Astronotus come from tropical South America, and typically inhabit large, whitewater rivers. Astronotus ocellatus is found across the Amazon River basin in Peru, Colombia, and Brazil. The range of Astronotus crassipinnis overlaps this to some degree, and is also found in Peru and Brazil as well as Bolivia and Paraguay. The common Oscar has also become established outside its native range in a number of places, particularly Florida, where these fish are feared to have exerted a negative influence on the populations of native sunfish, such as bass, that occupy similar ecological niches. On the other hand, Oscars are excellent sport fish being powerful and of good size, and at least some populations of Oscars were established through deliberate introductions of juvenile fish into waterways by anglers. As with other cichlids though, their intolerance of low temperatures restricts their ability to spread further north than Florida.

Both species of Astronotus are fish of sluggish waters, and spend most of their time in and around sunken vegetation. They periodically rest on the substrate, and unless actively foraging do not move about much. They are eaten by a large number of predatory fish, birds, and mammals, and their cryptic colouration helps them to blend in with their surroundings. The eyespot on the tail provides a second line of defence, directing predators towards the "wrong end" of the fish, making a successful escape much more likely. There's also some evidence that the eyespot confuses fin-nipping and scale-biting fish such as piranhas.

Astronotus spp. are closely related to the acaras and eartheaters, and to some extent share the same opportunistic foraging strategy, sifting through sediment and detritus to find small invertebrates such as worms, insect larvae, and snails. But they are able to handle much larger prey items as well, and have powerful pharyngeal teeth that are used to crush any spines or shells before swallowing their prey. Armoured catfish such as Corydoras as well as large snails, crayfish, and freshwater crabs are all important parts of the natural diet of Astronotus cichlids. Less frequently, they will take small vertebrates such as juvenile terrapins, and at certain times of the year plant material such as algae and fruit may make up a significant part of the diet as well.

Behaviour and breeding

Oscars have been bred in home aquaria for many generations, and consequently the social and spawning behaviours of this species are very well studied. Males and females are notoriously difficult to recognise, though among wild fish at least the males tend to be a little larger and more brightly coloured. Some aquarists have coined various rules-of-thumb for sexing juvenile Oscars, but these tricks are far from foolproof, and the best way to get a mated pair is to rear a group of juveniles together and then remove any excess fish once a pair have formed. Getting the bond between the male and female is important, as simply putting a male and female together from different tanks and hoping for the best generally doesn't work. These fish are territorial, and interlopers into a certain fish's home range will simply be viewed as a threat, irrespective of its sex. Oscars are basically placid animals but they can be fighters when sufficiently motivated, and rival fish will threaten one another slap each other with their tails, and engage in tug o' wars by locking jaws and pulling. Rival males will fight over territories most vigourously, but males and females will fight too. When forming a pair, a male and female that are interested in one another will also engage in threat behaviours and shows of strength, though by comparison with actual fighting such behaviour tends to be rather mild.

As with most other cichlids that visually appear very similar, Oscars are biparental spawners, sharing the tasks of defending the nesting site, cleaning the eggs, and defending their brood more or less equally. The are extremely prolific fish in terms of egg production, and will produce several thousand eggs per batch. The eggs hatch after about two days and the fry become mobile within the next six to seven days. Being fairly large, the fry are quite easy to rear on powdered flake food as well as things like newly hatched brine shrimp and small daphnia. Oscars are somewhat unreliable as parents, with some pairs being very diligent right from the start, but other pairs need a few trial runs before they are able to successfully rear a batch of eggs.

Even if the parents manage to care for their eggs properly, after about a month most aquarists remove the fry and rear them in another tank. This makes it easier to feed the rapidly-growing fish without harming the water quality in their parents' tank, and it also makes it possible to cull any deformed fry. Baby Oscars can be cannibalistic, some separating out the larger ones from the smaller can be useful (among cichlids generally it is not uncommon for females to grow more slowly than the males). Regular meals and plenty of water changes are the key to optimising the growth rate and health of the fry. Selling hundreds of baby Oscars isn't easy, and it is very important to start off with top-quality fish of a single variety so that you can entice your local aquarium shop to take the juveniles off your hands once they reach a marketable size.

Aquarium care

The maintenance of either species of Astronotus in captivity is not difficult, as these are essentially robust animals. Oscars do like the water to be quite warm, especially for breeding; 24-26C for regular maintenance and 28-30C for breeding generally works well. Of course, when water temperature goes up, the solubility of oxygen goes down, so ensuring that there is plenty of water movement and additional aeration if necessary is critical. Water chemistry is not particularly important, though in the wild they come from waters that are soft or only moderately hard, and the pH is slightly acidic to neutral. In Florida the water is often quite hard and alkaline but the Oscar has managed to thrive there nonetheless, and in aquaria Oscars have been found to be quite adaptable as well. Water quality, on the other hand, is critical, and these fish must only be kept in fully cycled aquaria with plenty of filtration. Aim to install one or more external filters that turn the water over up to ten times per hour (in other words, for a 100 gallon tanks, use a filter with a turnover of 1000 gallons per hour). Nitrates should be kept as low as possible, something that is complicated by the fact that these fish are greedy and messy feeders. Regular water changes are important, certainly in excess of 50% per week. The filter will probably need to be cleaned on a weekly basis as well.

Oscars are notorious for reconstructing their environment, and will dig trenches in the sand or gravel, uproot plants, and pretty much move about anything they can carry. Glass objects such as heaters and thermometers are particularly at risk from being broken in the process. Filters with built-in heaters are the ideal, but otherwise the heater should be securely protected with a plastic cage so that the Oscar cannot reach or damage it. External thermometers such as adhesive LCD thermometers are a cheap and easy to use alternative to traditional glass thermometers. External filters are at risk from having the siphons and return pipes moved about or clogged, so these need to be installed securely and in such a way that they cannot be pulled loose.

With regard to substrate, a thin and easily cleaned bed of sand or fine gravel works well and allows the Oscars to dig and root about for food in a natural way. Because they are so large and messy, the substrate should be cleaned each week by raking it through and siphoning out any detritus. A few large rocks and pieces of bogwood can be added as well, and this will give the fish some territorial boundaries as well as hiding places. Floating plants are also useful and are generally ignored. Some aquarists have had luck adding sturdy plants attached to wood or rocks, such as Java ferns, but this is always a gamble and many Oscars simply uproot them.

As far as tankmates go, Oscars are safe with any other placid fishes too large to be eaten. Plecs, clown loaches, tinfoil barbs, giant gouramis, and large spiny eels have all been kept successfully with these fish. Temperamental cichlids, on the other hand, tend to be a less worthy choice because Oscars are, despite their size, rather mellow and do not do well if forced to fight for their survival on a continual basis.

Oscars are big fish. A school of juveniles being grown on together will need at tank around 100 gallons in size, and a pair will need at least 50 gallons of space. Astronotus crassipinnis is smaller and can perhaps get by with less space, but still, these are not fish for aquarists with limited room to spare.

Diet and health issues

As indicated earlier, Oscars will eat a variety of foods and in captivity at least are easily maintained on pellet foods, whitebait, mussels, and frozen invertebrates such as krill. Wild-caught Astronotus crassipinnis may be a little fussier at first, and live foods such as earthworms and river shrimps can be used instead. Eventually though, even these will learn to take alternative food items, as both species are intelligent and quick to recognise their owner, and once they know who feeds them, they'll eat pretty much anything offered. Oscars do eat some vegetable matter in the wild, both directly and indirectly through the stomach contents of the animals they eat. Vegetarian flake food and pellets are usually accepted readily and make a good dietary supplement, while mussels often contain a lot of algae and make a very good addition to the diet. Most Oscar breeders do not recommend the use of live feeder fish because of the risk of introducing parasites and infections, and using feeder fish is of doubtful legality in the UK anyway.

Oscars are particularly prone to Hexamita infections. Hexamita spp. are protozoans that infest the digestive tracts of fish, particularly cichlids, and in the process causing the fish to lose weight and gradually waste away. Eventually, sick fish will die. The only effective treatment is the use of the antibiotic drug metronidazole, obtainable on prescription from a vet. Over-the-counter treatments from aquarium shops are generally not very effective, though they may have some use prophylactically when used in the quarantine tank immediately after new fish are purchased. Healthy fish sometimes carry the parasite without showing any symptoms, and there is some evidence that poor water quality is the thing that tips the balance in favour of the parasite allowing it to multiply rapidly and cause disease.

Hole-in-the-head disease is another common problem with Oscars, and again the cause is poor water quality. Hole-in-the-head disease is essentially a degeneration of the lateral line system across the head, the normally tiny pits turn into ulcers. Antibiotics such as metronidazole will slow down the disease, but the chief response has to be dramatically improving the water quality. Ensuring that the diet is balanced is important as well.

Final remarks

Oscars fall comfortably into the category of "pet fish", each specimen developing its own personality. Given that these fish can live for well over ten years, many Oscars learn a variety of tricks (as well as bad habits) and provide their keepers with plenty of entertainment. Though perhaps rather more demanding than their wide availability and low price would suggest, the Oscar is an excellent fish for the ambitious aquarist prepared to expend the time and energy on this fish that it requires. Though rarely seen, Astronotus crassipinnis offers all the pluses of the Oscar in a smaller package, and is perhaps even more desirable, and certainly well worth looking out for.

At the time of writing this article, Wildwoods have four specimens of Astronotus crassipinnis available. To secure any of these fish we recommend that you act quickly. To find out more about Wildwoods and to view their full stock list click here.

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