I have used gutter to make a cover for the bare light bulbs. It is not the right shape (parabolic) to make a good relector, no matter what treatment you do. I stuck 'peel and stick' mylar tape to some of them and spray painted the others with a silver paint.
At least they shade the light from entering the rest of the room, but I think more of it is absorbed by the gutter rather than getting redirected into the tank.
So... if you have overdone the lights, and simply want something so you are no blinded by the light that is sitting on top of the tanks, gutter is the way to go.
I got the stuff that is brown on the outside, and it looks better than the white, but I have not seen it lately.