This place has great fish. If you want a good selection, you have to go to the Marché Aligre fairly…read moreearly (11 am should be fine) if you want to really get your pick of good seafood. Overall the service is great (not American - is there anything I could possibly do for you - great, but great for Paris). :D
They have tons of choices, including oysters, "bio" options like salmon and dorade (sea bream), whole shrimp and scallops, and tons of whole fish (both wild and farm raised). The employees can help you pick what kind of fish you are looking for and then if it requires preparation they will send it to the back where there are about 3-4 expert knife handlers that will prepare your selection to your specifications - well that is if they are not completely swamped. The prep time can be a bit long but normally by the time you pay, your order is ready. (Plus the accept credit cards! - when the machine works...) For example, I got my dorade prepared for sushi and they took the whole fish and sliced it up nicely for me.
Also, they bought out one of their competitors at the end of rue Alige and have turned that shop into an oyster bar where you can get a plate of a dozen oysters and some white wine and enjoy yourself on a little table in the open air.