lol I don't have any fish smart enough currently. I did have a SW setup with a clown trigger and picasso trigger that loved being petted. The petting stopped when I added two red volitans lionfish (150 gallon acrylic)
Well my BGK isn't as petlike as that...but that's because I have't had enough time to hand feed him his blackworms (he's still small) and I have heard that they will do a "happy dance when fed"...they're like underwater ferrets
I think the main thing is that with other fish, they know you with their eyes...so they know when they are fed (your torso wavering in front of a tank, followed by the tub of food coming into sight) whereas the BGK's don't actually see you that well (or at all) and either know it's feeding time by smell or when their electrical field senses that a "tubelike thing" (your arm) waves food in front of it... and if you use a siphon, the sucking end resembles your arm. Just a personal hypothesis...
My aro plays with me and swims around my arms when ever I clean my tank or re-arrange the driftwood. I poked my Fat S.Tiger when he bullied his tankmates and ate the three clown loach lol
I hand feed and when I had an oscar it swam up to my hand. But when I hold my hand very still or sometimes not so still my knife fish swim around and tickle my hand.