The Oceanarium is one of the best facilities of its type.  It is well laid-out, very well maintained, and has excellent specimens in the viewing areas. It is also quite easy to get to on the Metro - take the red line to Oriente (the last stop on the line), go through the new shopping centre above the station, turn right and its a few hundred yards down the road on the left. It's not well signposted, but if you ask you'll find it. An interesting centre and well worth a visit.