Fish Fish and more Fish is off 27th ave, small shop but nice guys and what they lack in selection that make up for with cleanliness
6th Ave aquarium- its actually on Clement in between 5th and 6th in SF. Its a total rathole, complete with rude salespeople and a funky smell, but if you know what your doing they have the best prices. But know what your looking at if you go there. They dont have the best practices due to such small shop and crazy volume. Also check out thier Sunday morning bargain sale. They only keep items in the store for one week, so on Sundays at 10am when they open everything is all prebagged and discounted up to 75% to make room for the new shipments. Part of the fun about going there is who knows what they will have.