Answer by tvk for How to define the default vertical distance between nodes?

You can change the default by putting

\tikzset{node distance = 1cm and 2cm}

where the first parameter is the vertical node distance and the second is the horizontal node distance. If you only put one parameter, both the vertical and horizontal node distances will be set to that value.

As Trylks pointed out in the comments, in the node properties, use something like above= of 2nd1 instead of above of= 2nd1.