I picked up a 2" zapper once, and tried placing it in a 55 with larger cichlids in the hope they would learn to live together before it grew larger. The next day there were 1 dead, 1 dying, and 1 semi-stunned that died a few weeks later. It was moved into a 20 long by itself. Being fed on pellets it grew visibly in about a month, though not terribly fast. I didn't keep it much longer than that, gave it to a friend with a 6' tank and large Oscars. They quickly learned to leave it alone, and the dinner-plate size black pacu was terrified of the little bugger from the start. I'd estimate about a year to grow one close to full size.