Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Mine does too. I have a 20 long. It is in a hallway where a fan is blowing the air from the heater over the tank.
Lots of air movement above the tank = more evaporation.
In tank heater = warmer water, so it evaporates.
HOB filter = spillway, so it evaporates.
Glass lid, but I do not use the plastic strip that might cover the rear 2" of the tank.
Adding that plastic strip would slow the evaporation.