This is one of my favorite hobbies and I think it would be pretty awesome to design a game on the subject.
You could have a list of various components to choose from such as landmark features, plants, temperature, humidity, light, air-flow.... well, you get the point. Someone could choose a setup and then see how it does over time.
A game like this would be an excellent tool to teach kids and adults about ecosystems.
The same idea could apply to fish tanks. Along with the standard environmental factors you could teach how corals fight for space by stinging their neighbors.
Chemistry is a major component of the reef keeping hobby and that alone could be an entire game.
So, be it about fish tanks or vivariums I think a game on miniature ecosystems would be great fun and educational as well.