Thanks all,... when it comes to FULL spill gusto, a couple factors are in play.
1.) Pump-- A anemic pump will not be able to properly span your SPILL ROCK, it will dribble almost over the edge and fall back on itself. To big of a pump will erupt over the edge to violently, cause too much splash, potential leaks.
I always include a slide or ball valve AFTER the pump in a easy to get to location to be able to TWEAK the flow just perfect. For a little more $$, it's well worth having that flexibility.
Every project is different as head height to the biofiler-spill rock, how much media is in the bio filter before the spill rock ( too much or if it's clogged and needs cleaning can slow the flow) This project had a small run of 12 feet from the skimmer to biofall-spill rock. Used 1.5inch tubing on a 1500gph low pressure pump. The biofalls is a little under 2 feet from waterlevel.
2.)Tilt the SPILL ROCK. The wil ensure the water does not dribble back on itself, and increase velocity of the water going over it...
3.) Using small rocks placed on the SPILL ROCK (heavy enough for the water not to dislodge them, will push the water around them and make the water move over the entire SPILLROCK. Excellent trick to use if you did not level the SPILL ROCK correctly.