[pogodzib]

Many things affect evaporation rates. I'm currently evaporating 5 gal/day on my 220 gal tank. My old 125 gal tank evaporated 2 1/2 gal per day in the winter and 4 gal/day during the summer. 1 1/2 gallons per day isn't unheard of for a 55 gal tank. IMO, it is better to leave the tank uncovered and top off the water daily with an ATO device than to cover the tank, unless your house is showing signed of excess humidity in the air.