Under the Sea is in Cranberry, you will pass this on your way down 79.
Aquaword is also right off of 79 also.
Other stores that may be worth visiting would be Wet Pets in Peters (35 minutes south of the city, best fish selection IMO), Elmers Aquarium and Petland in Monroeville (30 minutes east of the city)
Are you staying in Downtown Pittsburgh? There really aren't any reef stores within the actual city limits that I can think of except perhaps the one in Greenfield who's name escapes me right now.
--ray