Sakana Sushi Bar is to be found in several cities around in Poland (at least 6 cities as far as I know). The chefs are trained in Japan, and their homepage claim that their Sushi is unmatched in all of Poland!
In Krakow you can find Sakana not far from the Main Market Square, so it is easy to drop by while in the center of Krakow.
Sakana Sushi Bar Krakow
ul. Slawkowska 5-7