If you're interested in corals, I've heard that Saltwater Paradise in Irving (O'Connor Rd.) is the best place to go in the D/FW
area. If you've got the money, Dallas North Aquarium seems to have a good selection, but I would be wary of the help and advice you get there. One place that surprised me was Pet-O-Rama (formerly Planet Pet) in Bedford (Harwood and Brown Tr). They had an extensive selection of saltwater fish, inverts and corals. I was only there for a bit, but the staff seemed to be pretty knowledgeable.