I've seen similar activity from mudpuppy-like amphibians and even tadpoles. Try taking a sample of what's in the pond. You've stocked FHM and these should show up in samples but see what else is actually there.
On one occasion in the mountains of Colorado I came across a pond that had that kind of activity. But no fish at all. Got all excited at first and tried to fish it. Finally, after closer inspection they were some kind of gilled amphibian very reminiscent of the mudpuppies we would get in our crawdad pond.
