Near the Parco Sempione, in the centre of Milan, lies the Acquario Civico: a very tiny Aquarium, on solely one floor and with an outside area, that opened for the first time in 1906. Though it is very small you can spend some interesting time in this place as it gives you a broad overview on the wildlife in the Italian lakes, rivers and the sea surrounding it.
The entrance charge is moderate with 5 € per person. And not only the aquarium is worth a visit but also the building itself, an old Art Nouveau villa with plenty of details to discover.
Viale G. B. Gadio, 2