In FM 0.9.0 beta 18 I found the usage of arrow keys and mouse wheel directions for zooming inconsistent.
Alt-Up is for zooming out
Alt-Down is for zooming in
(as was in 0.8.1) New in 0.9.0b18 is
Ctrl-Mousewheel Up is for zooming in,
Ctrl-Mousewheel Down is for zooming out.
I would consider it more intuitive to have the same directions for zooming in both cases. My suggestion would be to flip arrow keys in combination with Alt.