Hands down, the place to go is Neptune's.
1970 E County Line Rd
Littleton, CO 80126
(303) 798-1776
They are right behind Mervyns, which is going out of business. I think it's the very first shopping center after you cross University. They're on the left hand side if you're heading towards the mountains.
They have close to 100 tanks set up, with all kinds of fish. Salt water and fresh water set ups, and all kinds of corals too. If you want someone to offer you advice, without trying to sell something to you, this is the place to go.
Joe