They commonly reach 20 to 24 inches in captivity. I don't think a 30 gallon would do it. I'd say at least a 125, and a 180 would be better. They're heavy eaters, and with that comes a massive bioload. A larger volume of water, along with redundant filtration, would be needed to accommodate their waste production. Also, since they are capable of eating small fishes, their tankmates will need to be of the larger variety in order to be safe, and therefore, also carry a higher bioload. IME, they grow slowly, so these are max sizes that might take a while to actually happen. Mine has been fine in a 75 for three or four months. He's only grown about an inch, and that's with daily feedings from a turkey baster.