Cover reduce light?

[serratus]

Hum.... how much extra evaporation would there be without a cover? Say for a 50gal tank, how much are you topping up everyday to maintain same water level?

With the extra evaporation without a cover, are there a lot of hard to clean lime/salt/hard water grime that builds up on the inside of the glass at around the water level? or does that stuff come off pretty easily with Mag-Float cleaning pads?