Hi ...
Yes, there are indian fish shops but I can't tell you where exactly. But if you have some time, walk along Josefstrasse or Konradstrasse starting near the Hbf and you will see a sign for an Asian fish shop, also written in Tamil.
If you keep walking along Josefstrasse until you pass Langstrasse and Quellenstrasse, you will see even more shops. It's probably not a bad idea to familiarise yourself with this area as especially since the area around Quellenstrasse is turning out to be a Tamil area. On a saturday, you will notice cars from outside of Kanton Zurich and the shops get very crowded.
Agarwals on Langstrasse (near Helvetiaplatz) also has fresh fish. Possibly this'll be your first stop as it's the biggest indian shop around.
Good luck ...