Fishville is pretty simple, even by the standards of Zynga’s other games (Mafia Wars and Farmville for example). But this is exactly what Casual gamers are looking for in a game. You start with an aquarium. You buy fish eggs, hatch and feed fish then sell the fish. In the meantime you decorate your tank and have a limited interaction with friend’s tanks.
Varying your choice of fish eggs changes their feeding requirements and value. Whether you can play every hour of the day or for a few minutes every few days you’ll find a fish to suit you. If you don’t make the feeding times, your fish die and you have to purchase new eggs. You can easily get a few coins from cleaning a friend’s tank, so you are never completely stuck.
As you play you can expand your aquarium and buy a few items to brighten it up; they grant experience but do not affect the fish. You unlock different levels of fish as your progress each with their own value and feeding requirements. You can also unlock additional tanks, which allow you to neglect and kill even more fish.

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The game is currently in beta but seems to be stable enough. Future updates promise upgradable backgrounds for your tanks and an achievement system similar to... well almost every game being released now.
Other fish simulator games exist on Facebook, such as the highly popular Happy Aquarium from Crowdstar. It’s terribly similar and it wouldn’t be entirely unfair to say Zynga ripped them off but Zynga own the market for social networking games and can use their other games to promote new ones. I wouldn’t be that surprised if Fishville became the popular choice.