For ocean-life enthusiasts there is definitely a thing worth checking out when near the capital of Portugal. Lisbon Oceanarium (Oceanário de Lisboa) is actually one of the most popular Lisbon's tourist attractions.
I have visited a few Aquariums around the world before, but the Lisbon Oceanarium has left the biggest impression so far. After all it is supposed to be the largest indoor aquarium in Europe.
I visited the Berlin Aquarium a year ago, featuring the AquaDom - the World’s Largest Cylindrical Aquarium. You can read a little something about that trip and the aquarium in one of these posts.
The Oceanarium is located in the NE part of the city, near the Parque das Nações (you can check out the map here). You can easily get there by city metro (you will have to walk the last few hundred meters) or using other means of transport.
Besides a large permanent setup there is usually also an additional temporary exhibit - during my visit it was dedicated to turtles. In all parts of the large building an environmental note was always present. This is especially great for educational purposes since there are many young visitors.
There are four water-tanks placed around the large central tank, each housing a different habitat. The idea is to have a separate water tank representing wildlife from world's four different oceans (North Atlantic, Antarctic sea, Pacific and Tropical Indian ocean).
One of the most interesting species there is definitely sunfish, rarely seen in aquarium environment. Also very popular with visitors of all age is a pair of playful sea otters (Eusébio and Amália). In total there are 450 species represented by about 16,000 individual animals.
Apart from various marine species there are also some other animals housed there, for instance quite a few species of jungle frogs.
The entrance fee is quite high but in my opinion still worth it (if I remember correctly it was 16 euros). Family tickets with discounts are also available. You should reserve enough time for this - if you take it easy it might take you most of the afternoon to see it all. The Oceanarium is open 10:00 - 19:00.