Film over tray?

In ebb and flow is it best to cover the top of the tray with something that has holes for the plants so no light gets down there?

If so would black and white poly work?

you dont want light hitting the water ever really. light will dry up the water (but not the nutrients) so the toxicity level rises. that 1/4th strength nutrient solution will get stronger and stronger over time. also, you get salt buildups, youll see it caking up on the sides. another possibility, is you get algae, the green shit floating in your water. thats a plant, eating your nutrients, sucking your CO2 levels, and not leaving those for the plants.

Light on water = bad. use anything to not let it happen.

Cool thanks