Of all the fishes in the world, the Oscar comes closest in my estimation to the domestic dog... in terms of intelligence, capacity for learning, attractiveness, even loyalty to its owner. Astronotus ocellatus is a "stock item" in the aquarium hobby for all these reasons and more. Over the decades of its captive culture a few color (tiger, red, albino) and finnage varieties have been developed... and with them, increasing adaptability to aquarium conditions.
The bigger the better, for you and their sakes. Oscars should not be "started" or kept in anything smaller than a forty gallon for a single individual or sixty for two. Growth, behavior, vitality will be compromised in any smaller size systems. If you're interested in forming pairs, spawning the species, 125 gallons is a good starting point for growing out a group of six or so individuals, hoping they pair in the year, year and a half it takes to reach maturity.
Oscars have a wide tolerance of water quality range, but too often suffer from "metabolite accumulation" in their systems... due to being in too small volumes that are under-filtered, under-circulated, and mainly under-maintained in terms of frequent partial water changes. Really, about the most important aspect of their care is a very regular routine of gravel-vacuuming, removing of a good part (25% is about right) and replacement with new ready water on a weekly basis. Use the water on your house plants or garden... but make this exercise something you do every week. Of the principal environmental "diseases" of Oscars, pitting of the head (neuromast destruction) or HLLE (head and lateral line erosion) are directly linked to "waste build-up". Do these big, messy eaters their due, and change out their water regularly.
Oversized and tough are by-words here. Oscars are messy fishes to put it gently... and destructive toward objects like siphons and tubing in their systems. Outside power filters (more than one) are fine IF they are of the type that pull water into their boxes (versus gravity fed siphons), and even these are better retrofitted with suction cups on their in-tank parts. Canister filters of good design are fine, but must be regularly (as in weekly) removed for cleaning. Ideal arrangements for Oscar systems include outside sump type filtration fitted with external pumps and internal overflows and returns. Whatever mechanical means you set upon, make the filtration easy to maintain and over-size in terms of capacity and flow.
Should be in a word, "sturdy". Rocks, wood must be placed in such a way that when (not if) they are undermined (Oscars like to "re-decorate" constantly), they won't fall, break anything, like the tank. Heaters, filter parts, tubing... are all just play things to Oscars. Hide, remote, attach with suction cups... anything you want to stay in place... for a while.
Live plants... can be tried if they are exceptionally tough, the Oscars small... the system huge... but most folks utilize hefty artificial plants instead... and get plenty of wet-exercise re-planting them.
Should be selected on the basis of being hardy, fast, armored or if possible, all three! Most often encountered are other medium to large cichlids (hardy), tinfoil barbs (fast), and South American Suckermouth catfishes (armored), but other catfishes like channels and bullheads, Serrasalmine characins like Pacus and silver dollars, the larger Pencilfishes...
In the wild Oscars feed on small fish, crayfish, worms and insect larvae. In captivity, most all foodstuffs in all formats are taken with gusto. Aquarists can easily go awry with mis-feeding this species... as it will greedily consume most anything as food... It's important to vary the diet, and to provide foodstuffs of appropriate (smaller than their mouth) size... And to restrict oneself in terms of not overfeeding. Do consider mixing in foods of high nutrient and color-enhancing properties here. Along with frequent partial water changes, this is the best way of ensuring your fish's health and good color.
Don't fall into the "feeder goldfish" habit of stuffing your Oscars with these minnows. Not only are they expensive and almost always carriers of parasites, but goldfish feeding is nutritionally not the best route for you to go. Better to rely on pellets, sticks, cut meats (even beef heart and liver), eagerly accepted earthworms and occasional crustaceans (like grass shrimp, crayfish) as staples to alternate amongst.
Oscars are not easily sexed, and are even harder to "make" into breeding pairs by placing what seem like dissimilar sexes together. Here is another cichlid species that is best "paired off" by rearing a small population up together. Mature males are larger than females, and during spawning their genital pores make them easy to discern.
Mated pairs may produce several hundred young every few weeks for as long as a decade. Most folks keep such couples in a tank by themselves (to avoid destruction to other livestock), allowing them to spawn on a prepared hard surface, and either removing the fertilized eggs to another system for hatching and rearing, or allowing the parents to share in the duties of fanning, guarding their brood. Young hatch out in 36-40 hours at about 82 F. and are free swimming (wigglers) in about six days, and should be removed (if not raised separately) within a month to allow further spawning and reduce the likelihood of cannibalism.
As stated above, Oscars are tolerant of broad chemical and physical water make-up. Most folks keep theirs in water in the mid to upper 70's F., raising this to the low to mid 80's to induce spawning.
When small especially, Oscars can fall prey to protozoan infestations like ich and velvet, from which they are susceptible to common treatments. More problematical are flukes (flatworm) imports from live foods (mainly goldfish, other minnows) often employed by uninformed aquarists.
Looking for an aquatic equivalent of a family dog? Here's a fish you don't have to take for walks, that won't bark all night, and never jumps on the couch (though it may splash some water on it if its anywhere close to its tank). Oscars quickly learn to associate their keepers with food, and can be taught simple tricks like retrieving, lifting their heads out of the water, even rolling over for treats. As with canine pets, just be sure you've got the discipline to care for these lovable cichlids for the long haul.
Burgess, Warren E. 1974. The real Oscar rediscovered. TFH 2/74.
Kullander, S.O. & H. Njissen, The Cichlids of Surinam (Teleostei: Labroidei). E.J. Brill. The Netherlands. 1989.
Leibel, Wayne S. 1993. A Fishkeeper's Guide to South American Cichlids. A splendid survey of this attractive and diverse group of freshwater tropical fishes. Tetra Press, Blacksburg, VA.
Loiselle, Paul V. 1985. The Cichlid Aquarium. Tetra Press, Melle Germany.
Loiselle, Paul V. 1998. Oscar filtration. It's good to put your Oscar in a larger tank, but what type of filtration is best? AFM 7/98.
Loiselle, Paul V. 2001. Setting up an Oscar tank means space and more space. 10/01.