if you are in fountain valley, i would check out the following:
Tong's Tropical Fish & Pets (one of the bigger fish stores around)
(714) 842-2733
8976 Warner Ave
Fountain Valley, CA 92708
Pacific Reef (they concentrate more on salt stuff)
(714) 964-8899
18926A Brookhurst St.
Fountain Valley, CA 92708
Prehistoric Pets (if you are marginally interested in reptiles, amphib, etc, it is definitely worth checking out)
www.prehistoricpets.com
18822 Brookhurst Street
Fountain Valley, CA 92708
(714) 964-3525
if you want to go off the beaten path a little, 405 Tropical sometimes has interesting stuff (no plants though). I think people find the store unimpressive, but I really like it there. tetra are usually very cheap (cardinal, rummys, black neons, etc). the past couple weeks to a month, they've had what looked like Rineloricaria sp., Ancistrus sp., Nannostomus sp. (maybe trifasciatus?), a gulper cat, and they have interesting plecos from time to time. of course, E. Nishikigoi is less than a mile away so check it out. (for such a big place, it's easy to miss).
405 Tropical
(714) 373-0777
15222 Goldenwest St
Westminster, CA 92683
Eastern Nishikigoi (pond stuff, koi, goldfish)
http://www.enkoi.com/
14700 Goldenwest St
Westminster, CA 92683-5244