Mine hit the 16"+ mark by 2 years - he's in 75 gal. and it's too small of a tank. JP is correct 120XH 4x2x2 is a better life long tank than 6x 22x 18. After 2 years they do slow way down in growth however the girth of my fish is the size of my fist and forearm - there are pics of him in my gallery that were taken a year ago he's much wider now. Makes the 75 look like 30gal. Also his diet dramatically reduced over the past 6 mo. ( I've had him for 2 1/2 years) from 14-20 silverslides a week to 4. He is for sale as of the moment I have no desire to buy this fish a larger tank. Not the most active fish but very interactive and his size is awesome, makes ppl stop say damn!
And an FYI to the poster comparing 125 to 180 - the length is the same 6' the width and hieght are different.