SINGAPORE — Eugene Ng jabbed a pudgy finger against the side of the glass tank, like a predator singling out his unlucky target.

“That fish’s eye is looking a little droopy,” said Mr. Ng, pointing to a fish with large metallic gold scales swimming happily among its companions.

Minutes later, the fish was knocked out and getting an eyelift, a procedure that has become standard practice in Mr. Ng’s job as one of the premier cosmetic surgeons for Asian arowana fish here in Singapore. Using a pair of forceps, Mr. Ng — known to his clients as Dr. Ark, after the pet fish store that he also runs — worked quickly, loosening the tissue behind the fish’s eye and pushing the eyeball up into the socket.

“I know some people think it’s cruel to the fish,” said Mr. Ng, lifting his sedated patient with one hand to show off its newly straightened eye. “But really I’m doing it a favor. Because now the fish looks better and its owner will love it even more.”