I think a 36in long 50 is okay for a couple more years at least. The 18in width makes up for some room to swim in a circle, sort of. My bigger Oscar maxed around 14 inches after 8 years, the other was about 13. They had to point themselves up or down to turn around in that 55g, but they were always happy fish. Of course a bigger tank or a smaller fish would be recommended, but if you're dead set on an Oscar, just make sure you do what it takes to keep it healthy.
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