To be honest, it really looks a lot more like Yellowmargin trigger,
Pseudobalistes flavimarginatus then a Stone. IIRC Stone's get rather pale with size, and their heads become very blunt. Yellowmargins max out at 60cm, and I'm calculating the one in the above pic to be about 20"? There's also the namesake with the yellow margin on the edge of the fins...
Here are a couple pics I found. First pic a Stone Trigger, second is a Yellowmargin.