overflow help?

[kissman]

How can I tell what GPH this overflow is? I am getting ready to list it on ebay since I never use it anymore along with my Turboflotor T1000 skimmer and pumps.

http://bavasshokie.smugmug.com/Other/Aquarium/i-ZjSsRQn/0/M/overflow-u-tube-M.jpg

http://bavasshokie.smugmug.com/Other/Aquarium/i-Spt4njR/0/M/overflow-M.jpg