Evaporation depends on a number of things from tank size, sump size, how hot the lights are, the pumps that you're using, both the amount of heat they give off and the amount of water movement, and room temp, whether the room is hot, cold or humid or dry. All these things come into play with evaporation and depending upon the conditions you can have a lot of evaporation or a little. During certain times of the year people will lost a lot more water to evaporation then other times. For me when the temp is colder, and the room is dry, I lose more water to evaporation than when the room is humid.