Just thought I'd reply since the above post was for me - but I don't actually eat fish, so I couldn't tell you the best places, sorry
There's a place however that is off Langstrasse (across the big Denner, about a block up Langstr from the river, not the one on militairestr) that many people have mentioned to me. But I can't vouch for the quality, but if you're trying fish shops, that might be worth a try. I think it's called something like the "indian fish shop".
Happy "fishing"
~ Nanda