Minimum size tank for a couple of Oscars is 6 ft long x 2 ft wide x 2ft high.
They are predators and eat meat foods (fish, prawn, squid). They will also eat non-toxic insects and insect larvae. They eat earthworms and most frozen fish foods like brineshrimp, daphnia, mysis shrimp. However, as they get bigger you are better off using frozen bait for fishing (mulies/ blue sardine, white bait, prawn, squid, etc).
You can give them dry pellet foods for cichlids in addition to the frozen foods. feed the dry food first, then offer a bit of prawn or fish. Make sure the frozen food is defrosted before feeding.
Don't feed them live foods like feeder goldfish or guppies. It's not necessary and increases the chance of the Oscars getting worms and possibly Fish Tuberculosis.
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Because they are meat eaters and they eat a lot, they produce a lot of waste and you need to do big (75-80%) water changes at least once a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
You need to monitor nitrates and keep them as close to 0ppm as possible and below 20ppm at all times. If the nitrates go up, there's a good chance the fish will develop hole in the head disease.
The tank should be gravel cleaned every time it is water changed.
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Established filters should be cleaned at least once a month. An established filter is one that has been running for more than 2 months.