To the OP I've had my oscar in a 125 for two years and he's about 11" maybe. I haven't actually measured him out of the water. But I doubt he will ever reach 18". I also only say he cuz in the last two years it's never laid eggs.
Just saw this guy at LFS at least 18" could not believe my eyes. Poor guy was in bigger than a 55. But its all the store had and I'm sure its a rescue.
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bottom line is they can get to 18" this documented fact, most will kept Oscars will only reach 14 to 16" average over a span of 5 to 10 years. mines 1 year 2 months and a little over 10"