I poke him on the head every now and then to get him back when he tries to bite me and its just a big ball of water. He scratches and rips it very easily though so I have to be careful what I keep him with. No secret though, just genetics. His head always gets bigger after a day or two of not eating and gets smaller after a water change. Thats the only thing I can say about his nuchal.
Luck of the draw, waterheads show early, if you see a 2.5-3" fish with a round hump on its head you might just have one, but then again maybe not. Dad has alot to do with it. Hard KoK can get pretty darn big, but not as symetrical as a water is and nowhere near as big.
<------Hard KoK