My pond fish are disappearing - is it the frog?

I think that a raccoon, or some other night critter is probably getting the fish. Even a cat or a skunk could get a fish if it came right to the top of the pond. Putting out some kind of a deterrant is probably a good isea. Even one of those lights that come on when they detect movement would maybe help to scare off a marauding fish-catcher if it was sneaking in through the darkness, and everything suddenly lit up from the motion-detector light.
Unless the frogs are a lot larger than the fish, I really don't think that the frogs are eating them. Since you are asking about that possibility, you must think that the frogs are large enough to catch and eat the fish ?
Another possibility is that there is a turtle coming around to cool down in your pond. A turtle will most definitely eat the fish !