This is a well-done aquarium and a bit off the beaten track for visitors to San Sebastián (Donosti,…read morein Basque). Suggestion: start at far end of La Concha beach and curl around to the other end, where the aquarium sits (see map). You'll have to pass near some great tapas or pintxos bars, which is not a bad thing, ever.
Some of the fish seem friendly, others downright scary, judging from squeals of children (only drawback, in this enclosed space).
A great place to go on a hot day. I highly recommend it, for both the walk and the viewing of the exhibits. My favorites are the medusas (jellyfish) and the baby tiburones (sharks), which one can watch being born (see photo). I walked back to several areas to view again.