According to the Brits Guide (which I can highly recommend - fantastically informative book), quiet days at non-Disney parks are:
Universal Studios - Mon, Wed, Fri
IofA - Mon, Thur
SeaWorld - Mon, Tues
Busch Gardens - Mon, Tues, Wed
Have to agree with paulh, though, that August will be terribly busy no matter which days you go. Hopefully, though, if you go on those days it won't be quite as bad.
Have a great time.