The other reason is with bright lighting you will get a lot of heat retention. High water temp!!
Some want the tank to evaporate at a faster rate to allow additional top-off of RO/DI water combined with calcium and alkalinity additives. IÃ¢â‚¬â„¢m in a little bit of a different situation here in the desert. I have left a portion of the tank covered with glass to prevent or slow down a lot of evaporation (itÃ¢â‚¬â„¢s pretty dry here!) and prevent too much top-off from my Kalk reactor.