I have been to Waterworld in New Braunfels, and must say, it is a nice store. They have a nice variety of fish (not so much in corals). And they have called me several times on my request of certain. They also have a good variety of dry goods (lighting,sumps,skimmers etc.). I would recommend them.

With that being said, I think San Marcos would be a good location. I work in San Marcos and everyone I work with, though only a couple of people, recommend going into Austin for a good fish store. Setup a good shop, put your name out there and see what happens. Just my opinions, but that it why we are here. Good luck!