Hey Jim,
Is this for a new 125g tank ? An external C2C is dificult after the fact since you'd have to cut numerous slots/holes in back/top of tank. I have made a C2C w/ 2" pvc with about a thousand slots cut on a table saw and then hard plumbed through back of tank using 1" bulkheads, short section of 1" pvc into a reducing 2" - 1" pvc elbow that caps the 2" on each end. This is actually a very near C2C so you don't see the tipical C2C box open at the ends. I'll look at my old 35mm pics to see if I can come across any pics of it. Shoot me a pm if you need help laying it out. I also have a large table saw if needed.
Todd