First, it was sheep-throwing, then it was mafia battles, then it was farming, and now it’s fishing and aquarium-caring. If one is to judge society by trends in popular social games, you’d think the world is becoming a more peaceful, meditative place. And we suppose the social gaming industry is meditative, when it comes to closely examining competitors’ games. After a school of fish games emerged in the last couple of months (note: we will try to avoid the fish puns from now on in the article) big game developers have been quick to follow up. We already looked at Zynga’s new FishVille game last Friday. Today, we have another fish game: Top Fish, from Slide.

Launched earlier last week, Top Fish is the latest installment in Slide’s effort to move not so much into social gaming but in to social virtual spaces. The company’s first move in this direction was SuperPoke Pets!, a virtual pet-caring game, of sorts. The most recent is SuperPocus, a Harry Potter-style magical virtual world, but with cute virtual animal avatars to clothe and teach spells to. Top Fish has a similar dynamic to SuperPocus, and to some of the other fish games, like Happy Aquarium.

You start with a virtual aquarium, you get some free coins and buy a couple virtual fish, you feed them and watch them grow over the course of hours. You can do things like buy more fish, or decorations for your aquarium, or you can go and check out your friends aquariums. You can also get points for doing things like cleaning your own aquarium, or even your friends.

For serious fish-game fans, there are some big differences. Happy Aquarium, the genre leader with 24.7 million monthly active users and 6.93 million daily active users, focuses on fish care. It’s more like an aquatic version of a game like Pet Society, or SuperPoke Pets, where users name their pets, train them in obstacle courses, mate them to create more fish, etc. Top Fish, while young, appears headed in this direction. Slide has said it’s less focused on competitive gaming and more focused on fun virtual environments. FishVille, Zynga’s new fish game, is designed more like its virtual farming hit, FarmVille — the game is driven by the “farming mechanic” of buying, feeding and harvesting rather than slow-and-steady nurturing.

How will Top Fish make money? From direct payments, not offers. Indeed, you can see Slide’s in-house payment system live on the site. It’s a very simple window for entering your credit card, cleaner than the logo-festooned windows available from many third-party payment services offer. The company is also planning to launch a part of the store that lets users create and sell their own virtual fish, a virtual goods component that hasn’t been common on social games but has worked well for virtual worlds like IMVU.

But how does Top Fish fit into Slide’s plans? Here’s what Keith Rabois, Slide’s Vice President of Strategy & Business Development, had to say:

Inside Social Games: How will the user-created goods marketplace work that’s different from what other Facebook apps have done to date? Do you plan to use offers at all in the game?

Keith Rabois: As far as a marketplace for user-created goods, there is no real precedent for this on Facebook today. SuperPoke! Pets is probably the only major application that enables users the ability to create items that are sold in the Pet Shop.

No, Slide does not resort to offers in order to monetize applications. As Sebastien from Playfish eloquently explained, users pay for 100% of their movie tickets. If they truly enjoy our products, they will pay directly for the entertainment we enable. The best gauge of the level of fun, passion, and importance of an application is a user’s willingness to substitute his or her time and money away from consumption of other entertainment options, whether a movie, a DVD or an NFL ticket.

ISG: Given the changes that Facebook has made to its news feed and will make to other features like notifications, how do you plan to grow Top Fish? Cross-promotion? Ads on Facebook? How do you plan to take on the big aquarium apps out there?

KR: Slide plans to grow Top Fish the way we grow all of our other communities. We’re focused on building engaging experiences that people voluntarily share with their network of friends. We’ve always been committed to fostering long-term and passionate communities engaged in self-expression, content creation and interacting with other users. Because we emphasize retention of users for the long-term, we don’t confront the high churn rates of games and hence are not required to pump in massive numbers of new users every day to replace bored or unsatisfied ones.

ISG: In general, how does Top Fish fit in with Slide’s plans to build games, or “virtual entertainment spaces” or whatever the appropriate term is on Facebook and other social networks? I noticed the Top Fish app has quite an interface that’s quite similar to one or two other new Slide apps out there….

KR: The plan for Slide is for all our active apps to turn into consumer-driven entertainment economies.  Given that Facebook users clearly love their aquarium simulations, we decided to try our hand at a consumer-driven economy… in a fish tank. In our next few releases you’ll see some real divergence between what Slide are other developers are doing with our respective aquariums.”