Fins to Straighten Out Flight

Hi, This is Rich (Water Rocket)

I've mounted a camera on my rocket and it works fine now.

However my rocket "Rotates" as it goes up.  My Rocket is about 4inch Diameter  (a 2 Liter Bottle). 

Googling suggests that correct Fin design, etc is needed to straighten out flights. (along with symmetrical location and size).  

For those of you that may have LARGE DIAMETER rockets, can you please share with me what your fin design looks like, or advise me, etc.  Thank You