Nice fish.
I got one in Oct 2011 at 35 cm. The store had 2 in a 225 litre tank with lots of other fish including several young Phractocephalus hemiolopterus and several other species. The owner claimed he had it for a 2 year period. I put it in a 680 litre tank for quarantine and to get it used to my water and experiment on foods it would take. In time it took several different sinking pellets, cocktail shrimp, herring, mackerel, smelt, trout, tilapia etc. Then I put it in my 56,000 litre (15K g) until July of this year when I did some work in the tank when it was moved to a 1800 litre. It measured 58mm in July. While it does not give you a month by month growth rate it does give you the growth achieved over the 22 month period I've owned this one.