I do recommend you stay with the 55 gallon vs the 125 for grow outs since it's much easier to do large water changes on a small tank vs a larger one. During a water change, you have to factor in time for the water to drain out and to refill the tank. The 125 will take twice the amount of time to drain and twice the amount to refill vs the 55 since the 125 contains 2x the volume. Therefore, the entire waterchange process will take 2x longer on the 125 vs the 55. Unless you have the time to attend to the tank during the water change, I'd stick with the 55 for intensive growouts.