You O is being bullied. In the second to last picture you can see there has been scrapping going on by looking at the dorsal fin. It is tattered.
The slight pock marks on the forehead can indicate poor water quality.
A Oscar in a good quality Enviroment will grown one to two inches a month for the first six months when they pass there juvenile stage.
Ammonia/ nitrite are not present in a fully cycled tank. Nitrate becomes toxic for fish at 40ppm, toxic to humans at 50ppm.
These things are working against you boss. I have had my O for 8 years and we have had some tuff battles but she is still with me.
One O (13inch) in a 75 gallon planted tank. I know a little about O's cause I love em.
Get rid of the cons, do a couple w/c, turn off the lights, and listen to your fellow fish enthusiast. I would not introduce meds at this point. Change the Enviroment and see what happens. After that establish a good diet and work on feeding.
Ps we are all just trying to help.
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