Yes, Herons do take chunks out of fish.
Herons tend to land away from a pool then walk in.
But Herons tend to favor large pool and lakes, but have heard of them picking on koi ponds this time of year, my pool was cleared in a month by a Heron.
Nice fine if someone spots you flicking lead pellets at them too, be warned, I know of at least two people who have been caught 😉
I have worked at a lake where I have seen 20-30 Carp scattered dead around the lake & seen more with pecking dammage.
get a metal grid made up to protect them, make sure the grid is clear of the water by a good distance too.
We dont get stoats or mink round here thankfully so not sure if they could be your problem, but could be I guess?