Soupe de Poisson
Many years ago, I had a delectable bowl at the market in Nice. Which are the best places in Paris?
I've never found good provençale or nicoise food in Paris. It's enjoyable to cook, however. A soupe de poissons takes about 1,5-2 hours to cook, and is not very difficult. The good news is that it's easy to find the Mediterranean fish you need in Paris, even if we're much closer to the Atlantic.
I hope someone corrects me and has a lovely nicois restaurant to recommend; if not, are you staying somewhere you can cook? I can type up a recipe.
there is sometimes a stall selling excellent soupe de poissons at the Marché Saxe-Breteuil (on the av de Saxe near the place de Breteuil) in the 7th... but i haven't seen the guy in ages so maybe it's just a winter thing or he retired or they closed him down for poisoning customers (just kidding)
you can find very expensive and very good soupe de poissons at Le Dome Montparnasse ... but you could probably get halfway to Nice for the same price :)
there's a relatively cheap provençal restaurant C'est Mon Plaisir on the boulevard Montparnasse near la Closerie des Lilas and l'Observatoire ... the soupe de poissons is actually quite good ... under 20€ for a formule lunch ... and the tarte tatin for dessert is yummy)
don't tell anyone that i suggested it or i might get banned from this site but the very touristy brasserie Le Montparnasse 1900 on the boulevard Montparnasse/ place du 18 Juin 1940 has great soupe de poissons ... a little pricey but the decor is classified as a national monument so you gotta pay for the history too
quite elegant and in a superb setting, La Méditerranée on the place de l'Odeon in the 6th has superb bouillabaisse (soupe de poissons on steroids)... but i'm not sure if you can order it on the excellent-value "formule" menu (under 30 €)
but the best fish soup i ever had in Paris was at Le Petit Marius on the av George V near the place d'Alma in the 8th (a long long way away from your market in Nice) ... it's a very expensive place for a full meal but if you go late (say, 45 minutes before closing time) for lunch or dinner AND if they are not full AND if you are dressed to meet the maitre d's standards, you can smile your way to getting just the soup for 11 €
"Fish soup" is too broad. In Paris I would only be interested in La Marmite Dieppoise which is based on Atlantic seafood ( Norman). I currently have no recommendations in Paris. It requires quick attentive cooking and very fresh ingredients. My favorite place in Paris closed a long time ago. I suspect that now one needs someone like Le Squer at Ledoyen to make it to order.
Funny story I was actually afraid to try Soupe de Poisson when I was little because of the name. xD