Very good points!There seems to be a common theme here. And from the look of the tank in your first post, the water level is down a few inches from the top. That could be part of the problem.
The motors on hang on the back (HOB) style filters needs to be lower than the water level in the aquarium. If the motor is higher than the water level, there can be problems.
If you have a power failure, the motor will stop drawing water into the filter and the water level in the filter will automatically drain down to the water level of the tank. If the tank's water level is lower then the motor, the intake pipe will not be primed with water and the filter will not automatically refill when the power comes back on, and the motor will run dry.
External canister filters like Eheim and Fluval normally sit lower than the tank (although they can sit next to the tank) and are connected by hoses. If there is a power failure, the hoses remain full of water and the filter remains full of water. This means the motor and filter case are always primed and ready to start as soon as power is restored.
Have you had any power failures lately?
No power failure, and the water level was still above the motor, but this very well could still be the cause. Its an open top tank so evaporation is a pain in the butt with it. I have to top the tank off 1-2 times between my weekly water changes already.
That said, the water line is always 3 inches above the motor at the very least.