I made a smoke tinted cover to dim the lighting a little for my plecos and loaches in my 10G tank... it was only 1/8' thick and was supported completely around the perimeter of the tank, but it sagged a bit after only a month. I keep flipping it over every month or so. it seems to keep it in decent shape.
The fish seem to be more active with it on, but it still allows enough light through to see them very well.