Hi All, I'm new to the hobby and have a used 120 gallon I picked up off of someone. What I'm struggling to understand is how the overflow and sump work. Specifically, I'm trying to understand how, in the event of a power failure (and thus the return pump isnt pushing water out), the tank does not keep flooding into the sump until the water level reaches the height of my intake overflow. Below is a quick diagram I put together showing my basic setup. From the image, you can see the intake is below the water line, and gravity naturally pushes water down to the sump. Also, the tube from the intake overflow sits above the sump and just dumps the water down into the sump. What I'd like to understand is: In the event of a power failure, how do I stop the tank from dumping all the water that is above the intake hole? From what I understand, my sump should only really have to hold the water that's in my plumbing, but from my understanding of the diagram above, I think water would keep flowing down. Is there something I'm missing or overlooking? Any help is appreciated!!