Tank filling level
Check your tank for the minimum filling level for each film type.
One roll of 35mm uses in most tanks between 250 and 300 ml in standard manual development.
A roll of 120 uses almost twice this because the film is wider and the spiral at the wide position.
Don´t get confused with the filling level given for rotary processors. This is much less but if you use the conventional method your film will not be entirely covered by liquid and thus uneven development will result.