I am using a Dolphin DP 900 on my 125 gallon. (900 gph at 0 head, 17 ft head max)
I have two hang-on overflows with 1 inch pipes. I have 4.5 feet height to overcome, a swing check valve at the pump (McMaster-Carr online, check it out if you are interested), a tee that splits the return into two lines, one for each corner. My tank is 6 feet long, so I have about 10 feet of pipe on the return side. There are two elbows in each corner where the return lines go into the tank. All of my plumbing is 1 inch flex pvc or braided clear vinyl. I am using the pump as a submersible, in sump.
I was worried that my pump was not big enough, but in actuality I have to trim back the flow with a ball valve. More than just a little bit. This pump is very, very quiet and far more powerful than I thought.
I hope this helps
Good Luck,
-Christine