I think BGKs are some of the most playful fish. My BGK constantly does flips and will swim in my hand and then back up over and over again. I recently saw one at my LFS that wouldn't stop doing barrel rolls. It was so cute.
If a fish can experience fear and pain, why not joy? Why one extreme but not the other? I don't think they experience joy quite like we do, but rather comfort and content. If all their survival needs are met and they are placed in a stress free environment with good water quality and a healthy diet, I feel that qualifies as a happy fish.
Because of this thread I tossed a ping pong ball into my tank last night. So far, once they all figured out that they can't eat it they've ignored it, but I'll give it a few more days to see if they start playing with it.
u know i never rly paid much attention after i threw the ping pong ball in but everyonce in a while u here gulps and bangs and splashes from the tank im guessing they chase the ping pong ball. my husband is always trying to get me to take it out but its part of my sotck now oscar, gar, midas, ping pong ball....... Ps theres a video on youtube that has a owner tossing a ping pong ball in his tank and the fish (which i believe was an oscar) would hit back to him. and they would play like this.