Ditto on the Monterey Bay Aquarium. The Cal Academy in Golden Gate park (formerly the Steinhart Aquarium) has an aquarium, but it just opened recently and its reef tank, while very big was not very impressive in terms of its corals. I think in a few years once it has grown out it will be a very nice display. The Monterey Bay Aquarium is awesome, especially its huge tank of local marine life and the one million gallon Outer Bay tank, which is amazing. I have been there many times and never tire of it. Their coral reef section is not very big, though they do a nice reef tank, so if that's what you're after you might be a little disappointed. They have a new seahorse exhibit which I haven't seen yet. Its about a 2.5 hour drive from SF if you can swing it. Take the scenic coast route on Highway 1 if you can.