Two thousand Koi Carp, worth £20,000, were stolen from a garden centre in Kent over the weekend.
Staff said they thought the thieves would try to sell the fish
The fish were taken from Brambledown garden centre in Eastchurch, on the Isle of Sheppey, between 1700 BST on Saturday and 0700 BST on Sunday.
A number of dead fish were found at the scene when staff turned up for work on Sunday morning.
Staff at the centre said the thieves had taken "99%" of their Koi Carp and had touched very little else.
A spokesman said he thought the thieves had "done their homework" and knew where the fish were.
He said it was the second time in two years expensive fish had been stolen from the garden centre.
He said some of the carp stolen at the weekend were distinctive Butterfly Koi, which an expert would recognise.
He said: "With the amount they have taken, unless they've got an Olympic-size swimming pool, they weren't going to keep them."