So small is the poecilia sphenops, known as Molly, that any point of the 40 minute procedure, from being anaesthetised to the operation itself, could have proved fatal.
Sonya Miles, an advanced practitioner in zoological medicine, and nurse Laura Warren at Highcroft Vets in Bristol carried out the surgery, which cost Molly’s owners £100.
Ms Miles explained that fish must be anaesthetised in a different way to mammals, letting a solution flow over their gills before performing surgery.
She told the PA news agency: “We use a water-soluble anaesthetic, so we allow them to swim around in it until they fall asleep. We know they’re asleep when they lose their righting reflex, so they’re floating essentially.
“We catheterise their mouth and gently make different concentrations of an anaesthetic solution flow over their gills.
“With one that was less than a gram, it was pretty tricky.”
She added that Molly was the smallest animal she had ever operated on.
The the incision could not be closed by stitches as for a mammal – instead an adrenaline swab was used to constrict blood vessels, then a waterproof paste painted on to heal the flesh beneath the scales.
In a Facebook post detailing Molly’s operation, the clinic said: “We are pleased to say the fish made a full recovery and returned home the same day. The little one is now back to normal and eating well.”