The Mangarahara cichlid, described by zookeepers as "gorgeously ugly," is extinct in the wild and zoo officials said the hunt for a female is especially urgent since its two male fish are 12 years old.
In addition to the two cichlids in the London Zoo there is a single male fish in Berlin. The German zoo had a female in captivity but attempts to breed the pair ended in disaster when the male killed her.
"It's a fairly common thing with cichlids," London Zoo's aquarium curator Brian Zimmerman told BBC News.
London Zoo officials said a request to other aquariums around the world failed to come up with a mate, so they're now hoping a private collector somewhere may have a female.
Any collector having a Mangarahara cichlid is likely to be aware of what they have, Zimmerman said.
"They are not a particularly beautiful fish -- they are gorgeously ugly, they are unusual. They are more a connoisseur's type of fish.
"They need quite a bit of space; the males are bigger than your hand, and they need a decent tank," he said.
The species is named after the Mangarahara River in Madagascar where they were first identified.