When Another member in Colorado and I took a par meter to my 36g bf. we had very little loss between the glass and no glass top. However it getting crusted with stuff will block off some light. Every week during my water change I use a glass safe stiff sponge (petsmart has them) and some hot water and I clean off the top. You should not use any chemical on the lid as whatever you use will drop into the tank with the condensation
Thanks for the tip, I'll pick one up.