Are you doing a Durso, or a full siphon like a Herbie or Beananimal? I have a 36 gal with a Herbie style. My plumbing is 3/4" and I close the drain to about 75% capacity to balance with the return pump which is rated at about 570gph. A 1" drain should be able to handle well over 1000gph at full siphon.