Shooting further away from tank is okay for side shots with any lens assuming you have space to back up enough to "zoom" in or out as needed with your feet. Though, you'll get better shots with no reflection and glare if you press the lens (with lens hood) against the glass so that the lens is perfectly parallel. I'd be more concerned if you start taking top down through a view finder.
Here are some comparative pictures using a sensor with a 1.5 crop factor (the rebel is 1.6, I believe, so the resulting image would have slightly less field of view).
edit: oops. I don’t know what I was thinking, my camera does have a 1.6x crop factor, not 1.5. So the Rebel would see the same field of view.
60mm (coral in middle of tank - 9-10" from water line)
100mm (coral in middle of tank - 9-10" from water line)
60mm (coral on sand bed - 19" water line)
100mm (coral on sand bed - 19" water line)
60mm (coral on sand bed - 19" water line)
100mm (coral on sand bed - 19" water line)