I have one large Oscar in a 125 gallon, 6ft. long tank. The Oscar is about a foot long. The tank is very open. I don't have substrate, just a couple of medium-sized decorations placed in the center, against the back of the tank. We've considered adding another fish, but I'm hesitant because this Oscar is my favorite fish I've ever had. I've had this guy for 7 years and I don't want to add anything that will cause problems. Should we try it or just leave him alone? What would you recommend?