If you want a fishtank game, this is the one to have (though it's not properly a game, per se)
Fish are nice, and sprucing up your fishtank is pretty sweet too. What's better? Designing your own, no mess, no fuss, on xbox live. You have a decent amount of options here to design what it looks like, backgrounds, objects, kinds of fish etc. There's also a few examples programmed in which you can use if you're not big on creating your own.
But what makes this game even better is that you can design and paint your own fish! Pretty darn sweet - can't do that in real life can you?
Maybe including some sort of "game" here would have been interesting. Perhaps online fish contests, fish races, quests to obtain objects for your tank. As is, though, it works just fine.
Otherwise, there's not much to say. Turn it on and read a book or whatever you like. Enjoy!