I've read they can be lazy, but my Albino ACF, Takeo, is really rather active day and night. Of course he's been through a lot so I've been attributing it to never having had so much space all to himself before. He's also very easily startled and will zoom around from one side of the tank to the other when I move too suddenly, largely because he used to be hunted around his tiny vase by a curious cat his previous owners had in the house.
I bet if you just give yours some time he'll come around and swim more often. Activity can be part temperament part past experience. But I wouldn't worry too much if yours just hangs around the bottom of the tank a lot.