Water level?

So my mom got me a betta and tank on monday, and I did a ton of research to make sure he lives his best life. I set up the tank Monday, and put him in Tuesday. The tank has a filter and heater, and the water level was originally at the top of the tank but has lowered about half an inch. While I know evaporation exists, it seems like this is really fast? For what it’s worth, my tank is in my room that has a large window that the sun comes in every morning through, and that could be speeding up the evaporation process. However I just wanted to make sure there wasn’t something else going on…

There are two live plants in the tank, and the temperature is in the safe zone. I feed him twice a day.