Evap depends on a few things like water temp and air temp with the moisture (humidity) already in the air. So if you have warm tank say 80F and its winter time and your room temp is 70F and there is very little moisture in the air then your going to get a lot more evaporation than if your tank was 80F and the air temp is say 78 and its humid out as summers usually are. Now if your air temp and water temp are the same its going to cut down on a lot of evaporation, but the best way to control evaporation is to cover the tank 180G tank has a lot of surface area for evaporation so I doubt its a leak.