In North Rhine Westfalia, where Phantasialand is located, the Summer break ends on the 3rd of August. And, except for Oktober Fest and Rheinwiesen in Düsseldorf none of the Fairs attracts people that actualy stay a night or two. So fairs shouldnt be a big deal either for you.
But, the rates will be pretty high in August, as Menghini said, the Dutch are off and some other states in germany will also still be on holiday