3/4" on my 125g/100g top/bottom setup. but I have a valve on the pump inlet that i use for it, the pump does 700 gph at 4' of head (it's roughly 3.5' from inlet to output) so it drains the system pretty quickly.
My tanks are mostly barebottom and I have powerheads to push the waste into the overflow so there's no need for me to siphon it.