About two months ago, I set up an aquarium in my office. When fish starting disappearing, we put the most aggressive one, a zebra danio, into Fish Jail. When fish kept disappearing, we apologized to Hannibal and released him from the Finetentiary. (It appears that we had some supersensitive fish who died of stress, basically.)
No recent casualties, and two of the tank denizens are shown above; a glofish tetra and a platy fish.
Updated: Oops. I see I actually have three fish. Baby is in the corner.
(And all together now, “Nobody puts Baby in a corner!“)