I used 3/4 for fast drain back. Also used a horse troff float setup, they flow way more than the toliet ones. It used to take 45 min to drain back now it is 12 min for 24 gal recirc. Pump only runs for 4 min, so a complete recirc only takes 16 min. Dont think it matters, but I look at my nutes right after a recirc and I hate waiting so long for everything to drain back.