the greater the surface volume, how fast the water moves through the sump, fans cooling the water, the temperature of the apartment, and the humididty all factor into your evaporation rate. everyones is a bit different.
Joe i have a 120 gallon with a 30 gallon of water in the sump. im doing about 5 gallons every 2 days.... basements temp is about 75f and tank is at 80f.. not running any fans at the moment...
With this dry air that is around now, you might evaporate a bit more as well as the tank provides the humidity. Although that does seem kind of on the high side.
i evap about the same. the water level in my sump on the skimmer side is pretty high, about an inch from the top. i'm sure bubbles bursting at the top account for some of my lost water. i stay on top of my salinity levels because of this
Before I moved my 180 I was losing about 5 gallons a day. I was also running 2 fans over the open top tank and my basement was extremely humid. I also remember the tank running slightly on the warm side around 82 maybe even 83. Now I lose maybe only 1 to 2 gallons a day. I also run a chiller and I have no fans and the temp is about 79 steady.