After three days without food, the iFish began to steer himself better. He finally finished the one piece of betta food floating in the tank. He still prefers to lie on the bottom of the tank or lounge on the plastic plants, but his posture is more upright than before.
I dropped in a freeze-dried blood worm today and he went right to it. He took a bite, then resumed the position on his leaf bed.
His recreational swimming about has not yet returned to its usual level, but his behavior has definitely moved toward normal.
Moral: Overfeeding is just as bad for fish as for humans and other animals.