Built for the Expo98, the "Oceanário de Lisboa" contains a set of awesome aquariums.
Entrance costs only 10 euros and it is open all-week long.
There's even a project which lets kids sleep for a night next to the big aquarium (with a bunch of other kids and grown ups to take care of them).
The (public part of the) building is comprised of a huge aquarium in which you can see big fish, small fish, coloured fish, sharks and sometimes even divers, all at the same time. Around it is a long path, during the course of which you can see lots of smaller aquariums with other neat creatures (even a giant octopus and some sea dragons). And there are also other animals aside from fish, including penguins, other exotic birds and even two otters.
The big aquarium is really huge. And that is precisely the reason why you have to go around it and see it from different perspectives. Otherwise, you'd never be able to see all the kinds of different fish they have in there.
If you go during the week, and look carefully at the schedule they have outside, you can even see some of the fish being fed.
Just stay away from the piranha's tank
The limit Flickr imposes me is not enough at this time for me to upload all the photos I took there.
Nevertheless, here are some pictures.