I made my own perforations in the standpipes by drilling into them.
The first (working) standpipe had perforations from about 1/2 way up, all the way up to the top.
The second (emergency backup) standpipe has perforations only at the top two inches.
You use the gate valve to adjust the water level in the overflow so it is about 3/4 of the way up the working standpipe ie. about 1/2 way up the perforations on the working standpipe. Too low, and you get a loud gurgle noise. Too high, and it will overflow into the emergency backup standpipe. That is why a gate valve is needed, to get it adjusted just right.
http://reefcentral.com/forums/showth...hreadid=344892