The colourful koi carp, from a type originally bred in Japan, were stolen from a house in Parbury Rise, between April 18 and 22 while the owner was away.
The fish range in age from six weeks to nine years.
The haul, which included an 18-inch bronze koi and a 10-inch silver orange koi, were taken from the garden backing onto the railway line.
A tropical fish expert said that stress levels in fish have to be kept to a minimum, with the fish having a fair chance of survival if the thieves understand fish and offer them to a pet shop or introduce them to a garden pond immediately.
But he warned that if the robbers were casual thieves the fish are unlikely to survive.
The owner of the stolen fish, who kept them as a hobby, had some security measures in place.
Police say the theft would have involved quite a big operation given the size and numbers of fish.
Ring PC Janet Wheeler on 8247 5131 if you have any information.
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