water change---from sump or display tank

I siphon from the display and pump the fresh water into the sump at the same time

 

It's the level of the water, not the level of the vessels that's important for starting a siphon. The vessels can be on the same plane in space. The collecting water level must be lower than the source water level.


So, hypothetically, if you were using a siphon for your sump (which is on the floor), you could siphon most of it out if you had a particularly large footprint, yet not necessarily deep, pan.

 

I built one into my plumbing on my 120g build with its own"port" off of the return. Haven't used it yet, but we'll see.