Attached is my nano tank. My tank maker used black tinted glass for the DT/IOS divider, which I didn't like, but since its done...... so I can see the water in my IOS through the DT.
You can see that the DT water level is fixed by the overflow in the first chamber (left side),. You can also see the return chamber (on the right side) has a much different water level than the DT. This water level will drop with evaporation, not the DT water level.