For an aquarium game, Aquarium by DS has a nice suite of features. Containing over 30 species of fish, it gives you control over the type and number of fish in the tank, as well as food, temperature, water salinity, and decorative items. It even provides information about each species of fish, along with in-game information about each individual fish's physical and emotional state. So it's a pretty good aquarium game that actually gives you stuff to do. But it's still an aquarium game. The relative sedateness of the thing is, in fact, a selling point -- Aquarium by DS is part of the same Zen series as Paint by DS.
It's joined by Fantasy Aquarium by DS, which allows you to fill your tank with creatures that you would not be able to keep at home, like penguins, killer whales, and the noble platypus. Both games allow you to visit others' aquariums via Wi-Fi. But even with penguins and Internets, it's still looking at some stuff in a tank. The games come out in Europe in February, in case you will need calming down then.
Gallery: Aquarium by DS
Gallery: Fantasy Aquarium by DS