I'd start with a book like Understanding Exposure by Bryan Peterson. Scott Kelby also has some very good books for the beginning photographer. Unfortunately, there is no one "setting". It depends on many variables and changes with the conditions and the effect you're trying to achieve. The first thing you need to understand is how to attain the proper exposure. To do this, you need to understand how the camera captures light as well as how you can affect that through aperture, ISO and shutter speed. You can always put the camera in full automatic mode, and many times it will do a very good job, but without understanding these concepts, all you have is a very expensive point and shoot camera. Understanding these concepts can also help improve your photos with a point and shoot camera by the way.
Taking aquarium photos has some unique challenges, but you'll need to start with the basics, and I'd recommend trying to get a good grasp on those before even bothering to spend too much time taking pictures of your tank. I'm not saying that you shouldn't take some tank photos, but realize you're trying to run before you've learned to walk, so keep your expectations realistic. Once you have gotten the basics down, read through the stickies in this section, and of course, keep asking questions.