it has been years since i have personaly kept an oscar but i have kept several bluegill and sunfish and i would think that a 55 gal tank would be fine to support a single oscar, check this out here
http://www.oscarfish.com/
as for cloudy water I would use the process of elimination, if your:
1) water quality is good (constant ph, 0 ammonia, 0 nitrites, nitrate under 20
2) your feeding schedule does not leave food behind to feed bacteria in the WC
3) your filter's are adiquate for the tank size
4) you clean the tank at least once a week
5) is the tank close to a window where it gets to much sun? (algea bloom?)
6) is the fish eating like it should and is it healthy?
if everything checks out especily if the fish is healthy, then simpliy filter thge cloudyness out of the water.
I have a simmiler problem with my 40 gal native tank, everything checks out good but there is a slight haze to the water, i have ordered a diatom filter for like $70.00 (after tax and shipping) I plan on trying to run it once a week after my water change to "scrub" the water. after i get it i will PM you and tell u how it works
UNDERSTAND THIS!!!!
something is causing the water to turn cloudy in my tank and in your tank, I don't know what it is. I know that my tank has been stable (well i had a PH drop but i fixed that, i believe it was caused by agressively scrubbing a piece of drift wood and allowing tannins to get into the water) for quite some time and my fish are very healthy so i am going to "treat a symptom" and remove the haze by mechanical means. I would becarefull of adding chemicals to your tank that say they clear the water. Unless I knew how they worked.
my 2 cents
