i'm not sure how fast they grow as i've only had mine a short while but when it comes to activity they are very, very active. they almost never stop swimming. the only time i see them go to the bottom is to hide when people first walk into the room. after a minute or two they are back to cruising.
just be careful when getting more than one because they seem not to like each other very much. other fish they don't bother but each other they will chase and bite at.