Oscar

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: any.

Behavior

Plant interaction: destroys most plants.

Feeding

Will accept almost anything. Cichlid pellets are the staple. Avoid feeding feeder goldfish or rosy reds - they spread disease and lack proper nutrition; this advice is decades old and pet stores still push it

Compatibility

• Will eat any fish that fits in their mouth, which becomes a lot of fish as they grow
• Notorious tank-rearrangers; will move substrate, dig holes, push decor around
• Develop visible personalities and recognize their keeper; one of the few fish that arguably enjoys interaction
• Most-mis-sold large cichlid in the hobby. Sold as 5 cm juveniles to people with 75 L tanks; reaches 30 cm+ and needs 280 L+
• Compatible tankmates limited to similar-sized non-aggressive species: larger plecos, similarly-sized peaceful cichlids in 400 L+ tanks
• Tap water from a hose has been known to kill them via temperature shock; acclimate water changes carefully

Origin and habitat

Astronotus ocellatus, the oscar, is a large, intelligent cichlid from tropical South America, native to the Amazon and Orinoco basins and adjacent drainages, where it lives in slow rivers, lakes, and floodplains among submerged wood and roots. Agassiz described it in 1831. The tiger, red, and albino oscars of the trade are all selectively bred colour forms of the wild fish, and there is some suggestion the genus needs revision, so the aquarium animal may eventually prove to be more than one species. Oscars are among the most interactive of freshwater fish, often learning to recognise their keeper and beg at feeding time, and they are notorious for rearranging a tank, shoving substrate and pushing decor around. They grow fast and large: records give a maximum near 46 cm, with aquarium fish usually settling around 30 to 35 cm, and they are heavy-bodied, high-waste fish. Outside their native range they have become established as invasives in Florida, Hawaii, and Puerto Rico.

Breeding

A biparental substrate spawner that breeds readily once a pair forms, though pairing can take time and forcing a male and female together often fails. The pair cleans a flat surface and the female lays the eggs, a few hundred in a smaller fish up to a couple of thousand in a large one, and both parents guard them fiercely. The fry are free-swimming in about a week and are easy to raise on baby brine shrimp and then crushed pellets. The hard part is not the spawning but the space: a breeding pair needs a very large tank, and while guarding eggs they will attack anything else in it.

Common problems

Tank size is the overriding problem. Oscars are sold as small juveniles and reach around 30 cm within a year or so while producing a huge amount of waste, so a single fish needs a big tank and a pair more still. Hole-in-the-head disease, linked to the flagellate Hexamita together with poor water quality and a thin diet, is common and shows as pitted erosions on the head and lateral line; clean water and a varied diet are the prevention. Feeder goldfish are a poor staple, since they carry thiaminase and excess fat, so quality pellets, earthworms, and frozen foods are better. The fish are strong enough to crack heater tubes and shift equipment, and they jump when startled, so guards and a heavy lid are worth having.

Bioload

Further reading

• Fish that recognize their owners
• How bioload actually works
• Medication dosing for common fish diseases