Second, third: absolutely safe and much much cheaper than what you find in the high end pet food store. Comes in different sizes, too--big or small flakes.
I use those for topper's from time to time and also as a treat. My cats love bonito flakes, well most of them. Works for getting my elderly dog's appetite going too.
I use bonito flakes as toppers and there's never been an issue with diarrhea. You really only need a tiny amount. I doubt it's enough to cause any problems unless there's a severe allergy or something.