He's determined to squash these pesky fish.
They are equally determined that a large object touching the glass must be offering food.
They swim to his paws, he bats at the tan. They swim away, offended.
He swats at the bubbles from the filter, they swim back over to see if his paws bear food this time, he bats at them, they swim away.
He retreats, offended, until we leave the room. And then he jumps back up, and the cycle repeats. I hope he never realises the roof of the tank just lifts off.