Newbie question about covering the hydroton and reservoir

I did a few soil grows and will be attempting my first hydro grow shortly. In a lot of grows I see people covering their hydroton with foil or paper, especially when the plants are young. What is the main reason for this? Is it for better reflection or to protect the medium from the light which may cause bacteria?

My next question is about the reservoir. Should I have a lid covering the tote at all times to protect the water from the light?

Light tight the reservoir as well as the growing chamber. It's the same reasoning as covering the hydroton:

prevent algae/gunk growing in your medium/res. A reflective surface below the plant also improves reflection
of light back up to the plant.

So yes, definitely cover the res at all times, aside from nute changes/dumps and pH/ppm testing.

Covering the hydrotron also aids in reducing humidity in flowering. =)

Dont ever use aliuminum foil in any grow room. Increases temp and reflects almost no light. use flat white paint