Tough thing to buy used. You never really know what shape the membrane is in, and new filters are generally a must with a used system. Look at Spectrapure.com, they make the best you can get, IMO. Don't miss their refurbished units. Same quality at deep price cuts. Then look at BulkReefSupply.com. They have the line on economy set ups. There are other companies that sell this stuff, I mentioned the two I did because I have had good experiences with both.