Most oscars do not show HITH as juvies, or even in the first year, most not until at least the end of the 2nd year depending how off, the conditions like pH, elevated nitrate, hardness , overcrowding, or too small a tank are.
I have seen many posts , by people saying "my oscar has done great, and its been 4 months".
To me anything a couple years or under, I do not consider, valid experierience.
And certain fish are more prone to disease., than others, and more adaptable to adverse conditions, because of the conditions they experience in nature, choosing wisely is very important to a tanks overall success..
Those species not that far removed from pristine waters in the wild, placed in opposite conditions they have evolved to live in, should not be expected to thrive.
I collect fish from pristine, very aerobic rivers in Panama, where an 8.2 pH is stable within a couple tenths, and nitrate is undetectable.


Expecting these panamanian species to thrive under most most commonly accepted generic aquarium conditions, to me would be unrealistic.
So I maintain conditions in my tanks, that replicate the condition they experience in nature., as much as possible.
Below is my average parameter test, and flow.

