where I've been...
I'm from the Los Angeles area as well, the 310. I have gone as north as the valley and as south as the South Bay and can chime in on a few of the stores mentioned here. See previous posts for most of the addresses. I shop at these stores for their freshwater livestock & plant offerings. I get all of my equipment and dry goods online because it's FAR cheaper. Be sure to call ahead for hours of operation, because some have whacky hours! In no particular order:
NEPTUNE'S REEF: I second everything mentioned about the store. Great big plant & healthy fish selection, great display tanks of all sizes, helpful sales clerks. Clean, polished, spacious, well-kept. Great plant display tank.
MARK TROPICAL: in Studio City. I agree with the previous positive comments. Very nice plant and fish selection. A tad small and cramped, especially on weekends or sales. The tanks are clean though and clerks are attentive.
THE AQUARIUM: on Sepulveda in Culver City by Target. Owned by a really nice guy named Daniel. This place has come along over the years, with renovations and expansions. Decent selection of fish and increasing selection of plants (although some of the plants I thought were overpriced). There's a sign posted "no guarantee on livestock", BUT they did give me a few replacements on a sickly batch of Praecox rainbows I bought from them. The store isn't as "polished" as the above two merchants, but improving and recommended. Helpful clerks.
JIM'S EXOTIC FISH: on Sepulveda in El Segundo. Bought my tank, stand, and canopy from Jim. Nice guy. Clean, polished, roomy store. Nice planted display tank. Nice healthy fish selection. Modest, but healthy selection of plants. Bought some nice $7.99 crypts there, whereas another store wanted $14. Recommended.
ALLAN'S AQUARIUM: on Lincoln. Average selection of fish and plants; cheap prices, but not always healthy looking. I only check things out here occasionally. Sells more than just fish.
APEX AQUARIUM: on Sepulveda. Big, relatively clean store. Average variety of fish. No fish prices on most tanks though (big negative for me) and a little "cold" sometimes in the customer service department depending on the clerk, which is a double negative since you always have to ask how much a fish costs. Often have nice big Congos and Rainbows. Plant selection is small to none. Not a plant store, folks.
VEN-MAT TROPICAL FISH: on Venice. Used to be owned by Daniel of The Aquarium, but was recently sold off to new owners. Tank room is a wee bit dingy, tanks seem a bit run down and fish selection is weak. Plant selection is small to none. Pretty much off my radar now.
AQUARIUM CENETR: on Ventura Blvd. The website looked so cool, but the fish selection was not what I expected and smaller compared to other stores. Plant selection is minimal. Only place in LA that carried New Life Spectrum food though, so kudos to them!
ONANDA: on Santa Monica. Decent selection of fish I suppose, pretty much clean. I haven't been there in couple of years since buying a batch of cardinals from them (and the replacements they gave me) that dropped like flies in my old tank.
And yes, you might also want to peek at the occasional PETCO or PETSMART. Petco sometimes has some decent fish, the price is usually more competitive, and you can't beat their 14 day guarantee on fish. Just don't buy anything sickly. Petsmart might have a similar policy. Certain Petsmarts also price match web prices from other stores, so I was able to get 2 bags of Flourite for like $24 at the store in Torrance. Depends on the manager though!