At this point, it's unclear which company this is or why they picked FishVille. Chances are good that it is another competing Facebook company who might be working on an aquarium themed game. Many times, gaming companies will do in-house user testing to see what players enjoy and dislike about other games so that they can use the knowledge for developing the next great hit.
To participate, send an email to the address on the Craigslist ad, and make sure to include your current level in FishVille for consideration.
[via SF Bay Craigslist]