Staff at the Sea Life London Aquarium were baffled by the 4kg fish's refusal to feed, until they learned the gourami, named Gary, 40cm long, had been brought up on chocolate.
So the team stuffed crushed Kit Kat pieces inside grapes to get Gary to "take a break" from his daily fix.
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"I have never heard of a fish being fed chocolate, let alone being brought up entirely on the stuff," said Gary's handler, Rebecca Carter.
"Gouramis usually eat a diet of fruit but Gary doesn't appear to have suffered any ill effects from his chocolate addiction.
"However, we would not recommend feeding fish confectionery of any kind."
Gary is not the only unusual creature to come to the attraction from private owners.
Nearby lives Ed, a catfish with decidedly cat-like tendencies.
Ed will only accept his food by hand and likes to be stroked on the head at the same time.
He has also learned to head-butt a lethal puffer fish who vies for his food, staff said. Gary and Ed's interest in food highlights a more serious issue, aquarists said.
They have nicknamed one display the "tank busters" tank, because it houses a number of large fish that simply grew too big for their owners to manage.
"Many people don't do the right research when they buy fish and end up unable to care for them," said Ms Carter.
"Catfish are a good example and we have a number here that outgrew their homes.
"Terrapins are another - they can live for 40 years and are actually quite costly to care for.
"We'd urge people to think before they buy.
"We're delighted we could find a home for Gary but the fact is we simply do not have the space to accommodate the vast number of re-homing requests we receive."
