This is an old thread ---- I wouldn't recommend using flash from any angle, it'll just bounce off the glass back onto your film and you won't get a great picture.
Depends on the camera you've got what you can do really. If its a film camera, try to get a fast film then you won't need the flash. Using a fast film will also mean you can use a fast shutter speed so your fish won't look blurry. Again depends what kind of camera you have whether you've got any control over this anyway.
Get close to the glass to minimise reflection and make sure the glass is nice and clean. have a go, see what happens!