I have used Neptunes for years. They are very famous for two things: 1. The quality/health of their fish. and, 2. They are not the cheapest place in town. If you believe in the saying "You get what you pay for" then this is the place to go.
Sharon (owner) and Patrick (Manager) know their stuff. Try buying a fish and tell them you just bought a tank and are new to the hobby. They will give you lots of advice but WILL NOT sell you livestock untill you bring a water sample for them to test and then, only compatible and healthy (they quarantien ALL livestock) before they will sell it.
You will also get advice from everyone to purchase your own RO/DI system. I ignored all that advice for years and finally broke down and bought one. How did I ever get by before? Great investment.
IMHO the best of the best in denver is Aquamart, Keys Island and, Neptunes. (alpha. order).