I am not quite sure what meter we were using, kwirky would know. For the glass we tested it was not tempered and we were testing t5's.

the cons of a glass cover out weigh the pros, most reefers prefer open top it is better for the release of CO2 and the intake of O2 it also allows the tank to exhaust built up heat when Co2 builds up in your water column your PH will drop leading to your water becoming more acidic aquarium fish can live at 85 to 90 degrees fareinheit infact their natural habitats are this warm the reason you want to keep temperatures lower in the 70 to 75 range is to keep their metabolism lower thus decreasing the need to feed as often. you said you are new to this hence the big write up good luck and Welcome to canreef:wink:

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?