I think the OP is trying to address the glare or reflection from the glass. If the light fixture is mounted directly on the glass, then the visible reflection is minimized by the fixture itself. If mounted away from the glass (as in my case) then the reflection can be blinding. I solved this by placing a strip of black blotter board along the front of the fixture. Anything semi-rigid, thin, and opaque should work. Got this idea from Old Scales. He called it a light baffle.
In this pic, I have Finnex fixture mounted 3/4" above the tank but that is not clearly visible because of the black strip in front of it:
ignore the yellow circles