I am using a Rena Filstar XP3 on a setup nearly identical to yours. I'm willing to bet you a pet store person told you to buy that terrarium because your turtle only needed 4 inches of water, right? Did you go to the Wilkes-Barre PA Petsmart? (J/K, lol).
That's what happened to me, anyway. Unfortunately, since I wasted so much money on this lady's initial advice.(At least $250 wasted!!!), Squirt has to do with this setup for at least a year. He's 5 to 5½ inches, so I am technically 15 gal tank size too small, and it sucks that I've only got it half full.
With that said, I maximized my tank space by making a cork bark basking area that he can swim under, and the XP3 keeps it sparkling clean since it's such overkill, and I suspect it will do just as well on a 75 or 90gal tank when I get it next year.
For now, squirt is healthy and seems happy since this setup is lightyears away from what he was in at the pet store.
http://bover907.com/bover907/images/IMG_0817.JPG
http://bover907.com/bover907/images/IMG_0818.JPG
http://bover907.com/bover907/images/IMG_0819.JPG
http://bover907.com/bover907/images/IMG_0816.JPG
The lighting doesn't do the pics any justice, the water is sparkling clear and totally odor free. Only thing you can smell is the cork, since I only put it in last week. It has a sweet smell.