Oscar Fish

Much bigger than most people expect when they buy a small juvenile. A single adult oscar needs an aquarium of at least around 300 to 450 litres, roughly a 120 by 45 centimetre footprint or larger, and a pair or an oscar with tank mates needs 600 litres or more. This is because the fish reaches 25 to 35 centimetres, is heavy-bodied, produces a large amount of waste as a predator, and needs room to turn and swim. An oscar kept in a small tank grows slowly, develops health problems such as hole-in-the-head disease, and often dies young. Powerful filtration and frequent large water changes are essential.

Oscars are territorial and predatory rather than mindlessly aggressive. They will eat any fish small enough to fit in their mouth, and they can be bullies or be bullied depending on the individual and the tank. Suitable tank mates are robust, similarly sized fish that are not so aggressive as to harass the oscar and not so passive as to be picked on, such as larger plecos, silver dollars, Jack Dempseys, green terrors, or other large South American cichlids, all in a tank big enough that no one is cornered. Many keepers find an oscar does best either alone or with one carefully chosen companion. Breeding pairs become extremely aggressive and usually need a tank to themselves.

This is normal oscar behaviour and not a sign that something is wrong. In the wild, oscars clear and prepare nest sites by moving substrate and debris, and they also forage by sifting the bottom, so the drive to push gravel, dig pits, and shove objects is instinctive. Aquarium oscars often do it more than they would need to in nature, possibly from boredom or restlessness in a bare tank, and they will rearrange decorations daily, tip over pots, and bury heaters. The usual solution is to use a fine sand or smooth gravel substrate, few or no live plants, and only large heavy rocks and driftwood placed directly on the tank base or secured, plus a guarded heater.

They appear to, at least to the extent of telling a familiar person from a stranger and associating that person with food. Oscars are large-brained for a fish, curious, and highly food-motivated, and keepers consistently report that their oscar swims to the front of the tank and follows them when they enter the room, becomes excited at feeding routines, tolerates hand contact from its owner while flaring at strangers, and can be trained to take food from the fingers or through a hoop. Whether this amounts to a bond in the way a dog forms one is debatable, but among aquarium fish the oscar is unusually interactive and individual.