The municipal aquarium, near the Ponta da Praia at the eastern end of the city beaches, was the first public aquarium in Brazil when it opened in 1945. It is small by modern standards but well kept, with tanks of Atlantic and Amazonian fish, freshwater turtles, a small penguin colony and (in season) rescued sea lions.
Entry is free, which makes it one of the best deals in the city. It does get crowded at weekends; for a quieter visit, go on a weekday morning.