It is the old Nils Ericson's lock at Munkbron in Gamla Stan, which is being rebuilt to make it easier for fish that don't jump - for example eel, roach and perch.
Swimming past can be seen on fish TV
The cast fish walking trail will be covered and lie under Norra Slusskajen.
The plan is to install a fish counter but also install cameras to broadcast fish TV.
The screening will take place around the clock at Kornhamstorg.
The separate tier was a requirement made following a 2015 judgment in the Land and Environment High Court.