evaporation is a good thing. it helps keep your tank temp in check. and by encouraging it, you can cheaply keep the temp down without the use of costly and inefficient chillers and the like.
my tank is over twice the volume of yours and i go through 5 gallons every 48 hours.
if you dont want to deal with the process of topping off, look into auto-top off solutions.
glass on the tank will effect your lighting, and will inhibit gas exchange, especially if you dont have a sump.