This may go against the rule of "Dont tap on the glass" but I used to tap on the tank of my old guy and during water changes, I would chase him very mild w/the opposite side of my net. After about a month or so he didnt pay much mind to me touching his scales w/the pole. I'm guessing he got used to it (this is when he was about 12-13"). Each one is different but my guy was a little skittish when I first got him around 9". When tapping the glass, I would only do it when he was facing towards open water (length of tank).
Sounds kind of kooky doing it but I think it worked and could be one of the reasons I was able to keep a 16.5" Vittatus in a 8x30"x24" tall 300. But... he out grew my tank and sold him.