6" in 3.5 months is a crazy growth rate, even for an Oscar, how big was he when you got him? you can expect about an inch per month for the first year or so, PD is on right his size after a year. allot has to do with water quality and what you feed him. you can get him bigger per say with a high protein diet but this is not good for an Oscar and will shorten his life span, you are looking for a quality pellet with no more than 40 to 45% protein, after a year or so they really slow down, can expect little less than an a inch for the next year or 2, then allot less after that. there is a 22 year old Oscar in a Austria aquarium, he is 22 years old and 18 inches.