thats a real easy question to answer looking at the picture you provided
between the tank and the sump first off there looks to be very minimal water exchange if any . another reason is the sump cavity is about a 1/16th of the size of the tank the heater heats that 1/16th than shuts off a heater of that size is not designed to work in a small cavity like that anyway . I suggest you find a way of hiding the heater in the display . IMO
that sump design on back of the tank is a poorly designed and I wouldn't depend on it anyhow.