Ah, the top.
It's a glass cover of a picture/diploma frame (dollar store) 9x11 (glass is 9.25x11").
I hate evaporation, so I noted the dimensions of the tank and always kept my eyes open for a suitable top.
I even bought those plastic clips to hold a glass cover in hopes that I'd get a low-iron pane custom cut for me. But frugality won me over and I just use this (1/10 to 1/20 the cost).
I warn against using acrylic, b/c it seems to bend with minimal heat/humidity.
Its thin so not much loss of light, but im sure some light is reflected.
Hope that helps.