Agreed with everyone. Putting the skimmer in the sump will yield you the best results.
Also, Geoff's DIY skimmer is probably one of the best you'll find. Here's a link:
http://themantledmonster.thereeftank.com/sdiy.html
LOTS of people on here have made this skimmer, and had very good results.
Good luck, and WELCOME to TRT!! Coming here was the number 1 BEST thing you could do for your tank!! Everyone here is really helpful, and has a sincere desire to see you have a nice tank!
Also, one last question: This FW tank you're using...did you EVER treat that tank for Ick? most FW ick medications are copper based. Copper is extremely toxic (in any more than trace amounts) to most reef inverts.
Copper also has a knack for being absorbed into the silicone seal.
Basically, if this tank every had copper based meds in it, you run the risk of having major problems down the road in using it as a reef tank.
(just FYI)