You and your parents think about it for a moment. Then, Dad turned to the old man. "I'm interested." he said.
The old man thought. He motioned for Dad to follow him. He told you and mom to head to the tank view area so they could get a surprise. You and Mom did just that.
Dad and the old man were in a big metal room, that looked like the inside of a submarine, which had a bench nearby. The old man stood a few feet away from Dad. "Remove your clothing." he commanded. Dad was hesitant at first, but he did. He removed his shirt, pants, shoes, socks, and undies. He stood, naked, in front of the old man. Luckily the door in the room was already shut.
The old man smiled approvingly, then, he mumbled something even Dad couldn't translate. Dad suddenly felt a rather wobbly pain, and he gasped, as he saw his penis deflate, shrink, and get sucked into his body.
The old man motioned something else. Dad gasped as his feet became webbed, as did his hands, then he gained gills, then a back fin, he grew scales all over his body, his hair receeded, his legs became a fish tail, a second pair of fins sprouted from his chest, and his face formed that of a fish.
The old man smiled. He lowered Fish-Dad into the tank you and Mom were looking at. You and Mom were surprised to see Dad as a fish.