The only fish store I know of in Dallas is the North Dallas Aquarium. You might try Saltwater Paradise in Irving though. Its on O'conner, north of 183. They have a nice selection of both fish and coral. In the Fort Worth area, you might try Randy's Fish Warehouse, The Fish Place or Pet-O-Rama. I got my skimmer, powerheads, and lighting at Randy's. He has very good prices on dry goods.
-CuBB