If you put mesh or chicken wire over the tank, and find a hole in it, I would tend to think a wild animal got to them. If you do put chicken wire over the tank, spend the little extra to get the 1/2" by 1/2" grid, as if it is a snake, they would fit through anything larger. Not saying a garter can not fit through the 1/2" by 1/2" grid, but I doubt a snake that small would eat all the fish. If the top is open, it could also be the birds. As they are not dead and floating, something has to be getting at them. Some pictures would be great, especially if you can catch them in the act. Please keep let us know what you find out when you do. I am curious now.

Jason