My pond fish are disappearing - is it the frog?

I wouldn't blame the frog just yet. African clawed frogs will eat small fish when they get big enough but most frogs don't consider fish a prey item. There's a ton of other possibilities. Like Ram said, it could be a raccoon. It could also be a stray cat, a stray dog, a kingfisher or other fish eating bird or even one of the several species of bats that eat fish. Fish are like nature's treats.