O'Sullivan bought the fish, a ghost carp, about five months ago but it is only in the past week that he has noticed what looks like a mouth, nose and eyes.
'Because of the weather earlier this year I didn't really get to have a proper look at it,' said Mr O'Sullivan, of Dagenham, Essex.
'Then, last week, as I was sunbathing in the back garden I poked my head over the pond and there it was swimming towards me.
'It was astonishing. I could easily make out from the markings two eyes, a nose and a mouth. I thought I was suffering from sunstroke.'
Carp have long been known to carry markings which can look like a human face, but actually seeing one is rare.
As a result, O'Sullivan's fish is now said to be worth an estimated £40,000.
In South Korea too, a fish with a human face is making waves. The 'humanoid' carp are attracting attention in a small pond in the town of Chongju, South Korea.
They are believed to be hybrid descendants of two carp species – the carp and the leather carp, also known as a tangerine fish.
Both fish are females and more than three feet long. They appear to have distinctive human noses, eyes and lips.
The fish live in a pond behind the home of a 64-year old South Korean man and have been there since 1986 although their looks are only just starting to attract attention.
"My fish have been getting more and more human for the past couple of years," the owner said.
He added that he knows of other fish with similar features, and as they are both females it will be impossible for them to breed and have fish-faced offspring.
This is the second time in recent months that carp have made headlines in Asia.
Last November a shoal of the fish in Changsha, in China's Hunan Province turned on a duck who landed on their lake looking for food.
The Koi carp took exception to the intruder and fought back, bunching together into a seething mass to assert their authority, forcing the duck to fly away.
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