If you have a rising water current, say from a low outflow, fish tend to ride it for 'fun' and also because food in the wild often comes up on such upwellings and millions of years of evolution have combined fun, feel-good, and food for these fish. It's as important to keep upwellings from coming up near edges or hazards as it is to lid the tank, imho. But another, really good precaution is to rim the tank with a 9" false shell that's totally open on top [can support the light kit] and will dump any jumper back into the tank.