Sorry about that. Here is a link at marine depot for the check valve. All the check valve does is prevent the water from reverse siphoning into the refugium if the power gets cut which could cause the water in your tank to cause a flood. I had a eshops overflow box do that to me in the past. Eshops does make good overflow boxes. If you ever have the oppertunity to drill your tank I'd highly reccomend it though. It's much more efficient and quieter.
http://www.marinedepot.com/Swing_Che...FT8944-vi.html