Depending on your location, this time of year can often be very dry so evaporation will often happen much more rapidly. A gallon a day on a tank with a large surface area doesn't strike me as that much. In my area my tanks generally lose about 2.5 gallons every few days this time of year. I let them get a little lower than I might if I was "showing off" my tanks to anyone other than me, my girlfreind and our cats. The tank with an almost complete seal (glass cover, cutouts in back cut as close as possible to the components in back, heater, filter, air line tubing) does loss water less rapidly. If you have radiators to heat your home/apartment you could add a water pan to help humidify the air, or use a humidifyer. Either thing will generally lesson the evaporation from your tank(s). Also they will help prevent dry skin.
\Dan