All my tanks but one I keep just above the trim line... the only exception is the tank with my fire eel. I had a couple of eels pull some jumping exits on my tanks before, so I keep the water level a little more than a half inch below the trim. The tank has a hood, and I've just ordered an all-glass hood for it, but there are still some spots for escape around the filter output, heater cords, etc, so I leave a litte room so him aim isnt as good as it would be if he could just swim up to the opening.
J