It didn’t take long for the mystery to be solved. I caught our new-ish cat in the act:
She was drinking the fish water straight from the bowl. I think she was trying to get it low enough so she could scoop the fish out without getting her paw wet when she tried to fish the fish out of the bowl before I shooed her away. After I changed out the water and filled up the bowl again, she gave me a look like “Great. Now I have to get all waterlogged by drinking all the water again before I can reach my snack. Thanks.” Hopefully the fish won’t become a kitty snack anytime soon.