Tambaqui is a large freshwater fish that can be found in the Amazon basin.  It is considered the best fish you can eat in Manaus. 

The traditional way of preparing it is grilled, and the choice cut are the ribs (costelhas de tambaqui grelhado).  It is served in metal rods similar to those used in Brazilian churrascarias. 

Around 10% of the tambaqui's weight is fat, which gives it a wonderful flavor, similar to that of sea bass.  I had this dish at a restaurant called Canto da Peixada: a non-pretentious restaurant in a nondescript neighborhood, considered to be the best seafood restaurant in Manaus. 

Squeeze some lemon on top of it, and pour some freshly made spicy sauce (molho apimentado) and enjoy it with an ice cold beer.