If it's the Ghost Skimmer, they stink. Don't even waste your money. Yes, they make it for their own tanks, but it doesn't work well at all. Usually, just so you have an idea, a functional protein skimmer usually starts around $200 for a cheaper model. It may be better to look at the Aquamaxx HOB skimmers at marinedepot.com. They are cheap and actually work. The only thing is that it couldn't fit in the back. It would have to go on the sides.
I mean, it's only 40 gallons, and you could easily get away without one if you changed a 5 gallon bucket every week. Save the money and just stay on top of the 5 gallons a week and you'll be fine. My brother has 2 Ghost Skimmers on his Nuvo and hates them because they don't work well.