I've visited down there several times... not really LA area, but more in the OC area.
Coral Oasis - Pretty nice shop, they just relocated to a new place, so I think they are still settling in.
Age of Aquariums - This is my favorite shop so far, very clean with good fish and coral selection
Eye Candy Aquarium - I been here once, forgot how it was, think it was decent
Seaside Tropical Fish - Ok shop, decent size
OC Aquatic Tropical Fish - This place has a huge selection of fish, and pretty good selection of corals, this place is crowded a lot
Aquatic Heaven - This place is new, I called them, said they were open, I drove there, and sign on the door said, back in hour, out to lunch... oh well
Age of Aquariums is the closest to LA, and the one I like the most.