rallysman;1557215; said:Even if there is a glare on the glass, bubbles, dirt, or whatever, it's very difficult to crop out the background to make it appear to be black. A lot of the time the result will be a fake or obviously photo shopped pic. The best way to do it is experiment with lighting and shutter speed.
If the light on the fish reaches the camera before the light on the background (or if the light on the fish much more intense) it will underexpose the background making it appear to be black.