Less than a month we wrote that Facebook had just trademarked FishVille. Weeks later the company is officially launching FishVille. Happy Aquarium, the application which FishVille is based on, is now the 5th largest application on the Facebook Platform. Given Happy Aquarium’s presence, I’d expect to see FishVille advertisements all over Facebook.

So what is FishVille? It’s a game where you raise a fish, feed them and help decorate the tank which they live in. It’s similar to many of the other games like FarmVille, Cafe World, and Roller Coaster Kingdom, however in this one you are simply taking care of a fish tank. As you raise fish you can sell them to earn coins and experience. Honestly, it uses a similar format to all of Zynga’s other games which means the company could reuse a lot of the code from their other games.

In this game you simply grow fish, feed them, and then purchase items for your fish tank. Then come back once in a while to see how your tank is doing. Just like all the other games on Facebook, this one is passive so that you can check in a couple times a day rather than sit and play it non-stop. While I haven’t spent a lot of time playing this game, I’m sure that Zynga is actively tracking user metrics to ensure that this game is just as sticky as their other ones.

If it turns out to be a hit (which it probably will be), Zynga should blow past 200 million monthly active users, far outstripping any of the other competition on the Facebook Platform. If you want to spend your time building a virtual fish tank, go check out FishVille. Also check out Inside Social Games who first broke the news of this application.