The holes are: a 3/4" inch hole and a 1" hole in each corner overflow (holes cut into the bottom glass) which we have interpreted to be the 1" drain lines and the 3/4" return lines.

Then the right side glass has a 1" hole and the left side has two 1" holes, which we have interpreted to be one drain to the closed loop and a return from the closed loop on each side. I wish that I was one of those computer guys who could whip up a quick illustration in that sketch program so we could show you the whole elaborate plan but I definitely am not.