I am glad for you that there is an ending to your story and you have answers. It doesn't ease the pain of losing your fish but finally the mystery is solved.
We live on a farm where two creeks converge,, and because of this we cannot seem to keep chickens, turkeys, etc. DH has built the most secure shelters for poultry I've ever seen, but mink will slither in through the smallest hole and kill every one of our chickens. They don't eat them, but slit their throats
and leave them for us to find. DH saw the mink when he found the chickens, but of course didn't have his trusty firearm with him at the time...
So we don't try to raise chickens anymore.
Have fun shopping for new pets this spring!