I think it would be a mistake to add any more fish to your tank. A 55 is the bare minimum for 1 oscar and no other fish in the tank (a pleco would be okay though). Don't be fooled by his/her behavior. They get grumpier as they age, and don't take kindly to new roommates. If you've only had your 8 inch oscar for 5 days, it may just be pouting while getting used to his/her new home. It may be the food, or what you have in your tank. I think you would be headed for disaster if you added another fish right now. And if he/she is already 8 inches, it's probably too late to introduce a pleco. He/she would probably go after the pleco until it died of stress. I have 2 albino oscars and a common pleco in a 100 gal. tank, and they fight on a regular basis. They are probably larger than yours is. The one that always picks the fights has always been the aggressive one, and always gets his butt kicked. He even goes after my pleco which is over a foot long, but the pleco's sharp spines and body armor usually remind him right quick why he needs to leave mr. pleco alone. Give your oscar more time before doing anything else to the tank. I think he/she will adjust and become content with time. Just my opinion.
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