I've never been to Red C (though I keep meaning to), but I've been to a number of other stores in the Seattle area.
I don't mind The Fish Store and their new location is much cleaner and tidier. However, imo
the stock is less varied than it used to be and I've seen a lot of dead fish in tanks.
I can't recommend enough The Aquarium Co-op. The owner's great, my fish from there have been nothing but solid. It's more than a bit of a drive for me but totally worth it. They can generally get in just about anything that you would want.
Aquarium Zen, which is an ADA store, just opened less than a month ago. All of their tanks are planted display tanks. They're not well labelled, but they have good variety, beautful tanks and plants, and since it's basically a one man show, the owner can always tell you what he has.
Denny's Pet World is on the east side, and is more of a pet store with a fish section. That being said, I've never lost a fish from there within the first two weeks and I've bought more than just a few fish from them. And despite not only being a fish store, they have great variety usually, though less knowledgeable staff.
Blue Sierra's in Renton is nearest to me, but I wouldn't recommend them. Their tanks are almost always mislabeled and many of the fish I've gotten from there died immediately, despite apparent health. That being said, they do occasionally carry fish no one else does (I bought a trio of scarlet badis from the recently, including two females).
Tl;dr: Highly recommend: Aquarium Co-op. Recommend: The Fish Store, Denny's Pet World, Aquarium Zen. Don't Recommend: Blue Sierra.
Sorry for the novel. I'll definitely have to check out Red C.
Edit: I generally only do freshwater tanks though, and in retrospect, you were probably talking about saltwater. Sorry 'bout that