Just to clarify a bit;
The diameter of the return line has very little to do with the actual head pressure.
Head pressure is determined by the vertical height between two points ( the return pump and the spraybar in our case ) times the density ( specific gravity ) of the saltwater.
48 inches distance x 1.024 = 49.15 inches of water pressure.
If you took 3 pieces of PVC pipe each with a bottom cap screwed on - each 4 feet high and filled each with freshwater s.g. 1.00, measured each pressure at the very bottom of the pipecap they would all have the same head - 48" H2O.
Even if one piece was 1" diameter, the other's 2" or 3" or whatever ........
Pumping against these various return lines of different diameters using the same pump WILL affect each one's velocity a great deal.
The smaller diameter piping would have a much greater velocity compared to the other larger diameter pipes.
But the actual volume/time of water delivered back to the tank would be almost identical ( gph or gpm )
This may go against common sense, and may seem odd, but that's the way it is and there's not much we can do to change it.
3/4" or 1" I.D. is normally a standard size return line in this hobby - for most home sized aquarium set ups, although this is not always the case.
I would choose 1" I.D. return line(s) pipe or tubing with 1-1/2" drain(s)
