ANSWER: Well, Buddy, no one turned this guy loose from an aquarium. A common but rarely seen resident of that Nueces River estuary and similar habitats, it's known as the highfin goby, Gobionellus oceanicus. Until recently, the Gulf and South Atlantic population was thought to be a distinct species known as the sharptail goby, G. hastatus. The goby family is immense, with more than 2,000 species worldwide, and many thrive in incredibly diverse habitats. Members of this family are recognized by their pelvic fins, which are fused to form a suckerlike disc that helps anchor them in currents. Some even use this feature during spawning migrations to scale waterfalls in a series of forward leaps, punctuated by a break in between.
Even though the highfin tops out at only about 7 or 8 inches, it's one of the larger gobies. Most are less than a few inches. But what they lack in size, they compensate for in beauty, often with electric blue or striking yellow colors. Because of this, they are favored in the aquarium trade. So you weren't too far off-base thinking this fellow was an aquarium release, although it's devoid of the vivid features of many of its close cousins.
— Bob Shipp