I can appreciate wanting to cover your tank for this reason but be sure to consider some ramifications. For one, covering your tank with anything non porous will trap heat and will likely cause your tank water to increase in temperature. The act of evapouration helps to cool down the tank. Secondly, depending on how high your water level is, a tank cover can tend to develop salt creep over time as water splashes onto your cover. This will reduce light transmission from your light. You will find yourself having to clean the cover often if this happens. Anyway, these may be non-issues for you but something to think about just in case
