Actually seriously fish gives 35 centimeters as the maximum in captivity, and I am assuming this is not guiness world record maximum, either, with exceptional individuals being possibly an inch or two bigger, considering the igfa record was a bit over 18", or nearly half a meter.
While this was stated as exceptional even for a wild fish, fact is 2 pounds is considered a big fish, but attainable.
As stated before, Oscars are very sensitive to nitrate, and the first two visible signs of very low level nitrate poisoning in fish are reduced growth and reduced lifespan.
Other effects are anemia, elevated white blood cells and damage to liver and kidneys, although these require autopsies and blood tests to detect, to outward appearance the fish dies for no apparent reason.
Sorry, was mostly directed at the almost universal lowballing I see of potential maximum size for fish, and the fact that while it's easy to see when a tank is way too small, determining what is a bit too small can be tricky.