Often I'm asked how to avoid reflections when shooting through glass and in this QT I'll try to explain how unwanted reflections happen and how to work around them.
Red-eye and other annoying reflections are most often caused by the position of the flash and lens and their relationship to each other. Reflections from ones own flash occur when the angle at the reflective surface from the flash to the lens axis is 2.5° or smaller. I will refer to this angle as RA (Reflective Angle) See the example below.
If you have a fixed camera flash, the simplest thing to do is to move the camera lens as close as possible to the aquarium glass and thus increasing the RA. Unfortunately, moving the lens closer to the aquarium glass may bring the subject too close for your camera to obtain focus. Some cameras will accept screw-on magnifying lenses also called close-up filters which decrease the minimum focus range. This allows the camera to be moved closer to the subject while keeping the subject within the range of focus.
In the photo below, a flash unit has been added to the cameras hotshoe connector. Using the external flash increases the RA to greater than 2.5°, eliminating reflections from the glass.
