Hi Greyhound:
I am in the process of building a pond that is about the same as your dimensions, albeit double the depth. I just ordered this system:
http://www.cleanponds.com/product_info.php?products_id=212I dealt with Ted Lea. He is a regular on our forum. Very knowledgable and he was a pleasure to deal with. I suggest you contact him and start a discussion about the specifics of your situation.
Good luck with your quest.
Backstroke
P.S.
Where in New England are you? I'm in central NH.