Not nessiarcily. Just because 1 oscar grows slower than another doesn't mean anything. Each oscar is different, each fish is different for that matter. Just be sure to have clean water, which means good filtration, and weekly water changes around 35%.. But since you have 2 oscars and a pike, i would say 45-50% weekly to keep those nitrates down. Also might want to think about getting Chemi-pure. Stabilizes tanks, and can reduce nitrates down by quite a bit. Makes water very very clear. And be sure to offer a variety of foods, especially a High quality staple pellet like Hikari, along with some frozen foods, freeze dried, and crickets.