Technically thats not true Robb......the primary
should be able to handle the entire flow that the return puts into the tank, with the gate valve open all the way.....so if the emergency is the same diameter, then it should also be able to handle the flow if the primary drain is restricted more than the gate valve being closed to adjust the water line in the overflow......did that make sense?
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260g mixed reef, 105g sump, water blaster 7000 return, Bubble King SM 300 skimmer, Aqua Controller Jr, 4 radions, 3 Tunze 6055s,1 tunze 6065, 2 Vortech MP40s, Vortech MP20, Tunze ATO, GHL SA2 doser, 2 TLF reactors (1 carbon, 1 rowa).
http://www.canreef.com/vbulletin/showthread.php?t=50034 . Tank Video here
http://www.vimeo.com/2304609 and here
http://www.vimeo.com/16591694