I'd also like to add that even if you put a large growing fish in a tank too small for it, the fish will keep growing regardless of the tanks size to a certain extent (i.e. you aren't going to keep a 12inch long growing pleco or any other fish at 2inches long even if you put it in a very small tank).
A small tank may stunt a large growing fishes size to a certain extent, but this is very bad for the fishes health and will reduce its life expectancy and general health and quality of life. It is thus important to get your fish a tank large enough for it if you are to be a good and responsable fishkeeper- gallons isn't everything though, the actual width, length and height of the tank is also very important (the fish must have ample room to turn around in the when full grown and plenty of space to patrol the length and height of the tank) since tanks vary so much in shape and size now days.
If you cannot afford a large enough tank for the fish you want, simply don't get the fish. A 36gallon corner may surfice for a while for a juvenile oscar, but you will inevitably have to upgrade the tank sooner or later so you might as well get a much larger tank now if you want oscars
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