I would suspect you have a predator terrorising the fish, the actual tail fins of dead fish might be being eaten by other fish or tadpoles etc but the heads being missing would suggest something 'powerful'. Stress from regular predator visits might lower the fish's ability to fight off parasites and render them susceptable to attack. Did you look at any skin scrapes under a microscope, those have to be taken from a living or JUST died fish?
I would also suggest ringing the pond in raked or sprinkled sand and see if you can pick up any animal tracks