Take the night tour of alcatraz - truly amazing.
As for fish, there is a small fish store about 4 blocks north of union square... unfortunately I forgett the name of it, but it is in an ally way. I am sure if you head to the aquaruim someone will be able to tell you about it
You also M U S T see the sunset on Ocean Beach - amazing! There is an aquarium in Golden Gate park.. about 15 minute trip from the beach =)
If you can, eat at Mama's on washington square for breakfast. YOU MUST go to the bathroom, its worth it - food is fantastic... but you have to be an early riser. If you head there around 7am you can see about a hundred people doing tai chi in the square... and thats when the breakfast line up starts o_O Local artists have an out door gallery in the square as well.
And if you are the walking type, its just a 15 minute hike up to coit tower.. more tai chi, great view of the city, there is a small chinese herbalist up there too... mmmmmm I still have some white rabits I bought there =)
Have fun! San Fran is a fantastic place to vacation. It has a bit for everyone.