Well, having spent my college days in Davis and working at one of the aforementioned fish stores, here's my take on those three....
Capitol Aquarium. Decent selection of fish especially if you want to shell out the money for them. The salt water selection isn't too great and that goes for the quality of them. Definitely worth a trip just to take a look at that 1,000 gallon tank.
O. Street Aquarium. The owners (Jack and Peggy) are very nice, and once they know you, even the prices are negotiable
. Not sure if Shane works there now, but if he still does, I'd avoid him anyways, lol. The other employees there are pretty knowledgeable overall, as long as you're not probing them too deeply. Talk to Dennis if he's around and not too busy.
Exotic Aquarium. Another family owned shop, but in this case a couple of brothers (Nelson and Dickson). Of the three, I would recommend this over the other two. The selection of fish/corals/inverts are better than the other two and if they don't have it, you can get them to order for you pretty easily. If you're interested in saltwater, talk to Brian there, but if he's not there, talk to either of the brothers. For freshwater, talk to Mike there. He's basically the African cichlid there, but he knows a bit more than the others as far as freshwater is concerned. He gets his African cichlids from a local breeder and is also a member of the ACA himself. Check out the 300-gallon fish only tank as well as the two 240-gallon reef tanks.
Aqualife. Not actually in Sacramento, but further north in/by Roseville. Nice selection of saltwater if you're interested in that. Prices are a slightly above Exotic, but you find some decent deals there once in awhile. If for nothing, go check out their 1,000 gallon tank w/the white tip sharks and 3 foot plus green moray!!!
Sorry for the long post, but hopefully that was helpful. There's quite a bit more I can tell you about them. Just let me know if you want more info