honestly I'd question if this is even worth stressing over. 5.9 is above the EPA action level but it's not like you're living in a uranium mine. people freak out over radon numbers like it's the end of the world but the actual risk difference between 2 and 6 is pretty small in the grand scheme of things. plus those cheap digital monitors are notorious for being inaccurate, i'd bet money yours is reading high. before you drop another $300 on a fan i'd borrow a professional test kit or get a charcoal test done for like $20 and see what that says. if it still reads high after that, then yeah maybe get a second opinion, but honestly i think you might just be feeding into the fear factor here.