Macro mode simply allows the camera to focus on an object that is closer to lense than it could in normal mode. Say in normal mode you can only get with in a couple feet. In macro, you might be able to get with in a foot or closer, depending on the camera and focal length of the lense. If it's 18 inches away, your camera can probably focus on it w/o macro, but you may also have to stand back a little from the tank. In macro, your camera probably dosn't allow you to zoom in either, so it may seem as though you can't get as close. At least that's how my anceint 2 MP digi works.
