Plexi reduces light less than glass about 15% to 20%. Plexi is clear and glass is colored.Definitely.
Thickness isn't really relevant, the issue is surface reflection. A good rule of thumb is 10% loss for spotless glass under ideal conditions. In practice it'll be a bit higher. Glass is better than plexiglass, by the way.
Not having a cover on the tank definitely increases your water evaporation rate. I never had to add additional water between my weekly water changes on my 75, but now that I have an open top tank, I always add a 5 gallon jug midweek.thats great thanks! Gonna try without glass to see how things go....anyone find they have to top up water level before the water chanGE?