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

Native to South America, found in the Amazon and Orinoco basins, French Guiana, and the northern Paraguay/Paraná drainage. Inhabits slow-moving whitewater and blackwater rivers, lakes, and floodplains with abundant submerged wood, roots, and vegetation. Wild water runs pH 6-8 with tropical temperatures of 22-25 C. Tiger oscar, red oscar, and albino are among the most common selectively bred color forms; all are derived from the wild-type Astronotus ocellatus. The genus may need systematic revision, and the species in the aquarium trade may turn out to be a different Astronotus species. Renowned for intelligence and interactive behavior among freshwater cichlids. They can learn to distinguish their owners from strangers and associate familiar people with food. Known to rearrange tank decor, shoving substrate and moving objects (TropicalFishandAquariums). Can reach up to 40-45 cm and 1.6 kg; in captivity, 30-35 cm is more typical (TropicalFishandAquariums). Growth is rapid, which catches unprepared owners off guard. Lifespan of 10-15 years with proper care. Widely established as an invasive species in Florida, parts of Southeast Asia, and other tropical regions.

Breeding

Substrate spawner. Pairs (which can take months to form from a group; forced pairing often fails) clean a flat surface and lay 1,000-3,000 eggs. Both parents guard aggressively. Fry become free-swimming in about a week and are easy to raise on baby brine shrimp, then crushed pellets. The difficulty isn't getting them to breed; it's providing enough tank space for a breeding pair (500 L) and dealing with the aggression. A pair of oscars guarding eggs will kill anything else in the tank.

Common problems

Tank size is the number-one issue. Stores sell juvenile oscars at 5–8 cm. They grow to 30 cm in 12-18 months and produce an extraordinary amount of waste. A single oscar needs 250 L; a pair needs 400 L+. Hole-in-the-head disease (Hexamita) is common when water quality slips or diet lacks variety. It presents as pitted erosions on the head and lateral line; treat with metronidazole and improve diet and water quality. Their powerful jaws can crack heater tubes and damage equipment; use heater guards and secure everything. They eat anything that fits in their mouth, including expensive tankmates. Feeder goldfish are a poor food choice (thiaminase, parasites, fatty liver disease); use quality pellets, earthworms, and frozen food instead. Oscars are also jumpers when startled; a heavy lid is needed.

Bioload

Further reading

• Fish that recognize their owners
• How bioload works
• Stocking ideas by tank size