Blackpool is losing one of its oldest attractions to make way for a Lancashire version of the London Dungeon.
The Tower Aquarium is being revamped as part of new operators Merlin Entertainments' £10m re-development plans.
Work to move the 200 fish out of the aquarium starts on Sunday.
Most will be re-located to the nearby Sea Life Centre, which is owned by Merlin.
The rest of the fish will be quarantined until they can be accommodated at other marine centres throughout Europe.
Originally called Dr Cocker's Menagerie, the Tower Aquarium has been on the site since 1875, pre-dating the building of Blackpool Tower by 19 years.
Iain Hawkins, who heads Merlin's operations in Blackpool, said: "It is always sad when a famous old attraction closes but aquarium technology has advanced so much there really isn't any point in keeping the Tower Aquarium going.
"The transfer should be relatively straight-forward, but not all the fish are sure to go quietly.
"It's a safe bet that some members of the team will get a soaking."
Mr Hawkins said marine experts will take "military care" in moving the fish which range from tiny tropical trigger fish to huge South American pacu.
The Blackpool Dungeon, which will replace the aquarium, is scheduled to open in September 2011. It will feature the Pendle witch trials as well as a torture chamber and an ancient court room.
Merlin are managing the attraction on behalf of Blackpool Council who purchased the Tower, Winter Gardens, the Golden Mile Centre and Louis Tussauds in March.