When you worry more about what your feeding your fish than yourself. When you skip watching football games on Sunday to clean tanks and do WC. When you start turning your tanks sideways just to get room for more. When a fish room becomes the next home renovation priority over the leaky roof.
Almost of my tanks are vertical! I fit 6 5 gallons on a 48 inch and 4-5 on a 30 with board on the top! The only one that is horizontal is the one on the bottom of one of the racks that is a plants only tank that I plan to use to stock my other tanks.