I haven't been there, but I've heard good things about Jim's Exotic Fish in El Segundo.
As for places I have been to, Bob's Tropical Fish on Atlantic right off the 60 is a small shop and maybe not the cheapest, but they seem to be really helpful. My brother in law goes there regularly for advice.
There's CK Fish World in West Covina on Azusa Ave, just north of the 10. Not huge, but bigger than Bob's. Pretty good selection and they have been very helpful as well. My first trip there I was a noob (still am really) and I was looking at some not so cheap fish. They asked about my setup and when I told them I was just starting a fish in cycle they advised against spending my money until the tank was cycled. They could have just made the sale, but they advised against it. I appreciated that. I was just back there this weekend and found some horned nerite snails, rabbit snails and CPO's. Those are all things that I was considering, but I was thinking I would have to order online.
Just went to Global Aquatics in Ontario for the first time this weekend too. BIG place with lots of variety when it comes to fish. Seemed to have tons of cichlids, if that's what you are looking for. As with most local shops, the accessories seemed to be pretty expensive compared to the big chains.