Do a search, this gets asked all the time. Lots of people argue that the blue water carboys are too oxygen-permeable (as opposed to #1 type plastic, as in better bottles, which is not as oxygen permeable) and are not suitable for fermenters and may have some numbers quantifying the oxygen permeability of such plastics (which don't have much meaning to most of us), and you'll also hear from people who have used them for fermenters and never noticed issues. Feel free to read the existing arguments and make up your own mind.