Yes, I had an overflow box and sump setup. I have never had an issue with the overflow. Occasionally if you let the water level in the sump get low enough to start sucking air and it runs that way for a few hours you can get a air bubble in the return tube. At this point there is not enough water in the sump return chamber to cause the main tank to overflow. I just check the return when I top off. If it is running slow I just tap the return tube for several seconds and the water flow will draw out the bubble.
I've heard of people having a large snail get stuck in the return and cause an issue. But as long as you aren't overfilling the sump when you top off there shouldn't be enough volume in the sump return chamber to cause an overflow.
Just FYI that I am planning on listing the sump I used with this as well. I just haven't had a chance to dig it out yet. It is Eshopps RS200 that is about 4 years old. I'll try to get a picture tomorrow.
